Merge branch 'so/log-diff-merge'

"git log" learned a new "--diff-merges=<how>" option.

* so/log-diff-merge: (32 commits)
  t4013: add tests for --diff-merges=first-parent
  doc/git-show: include --diff-merges description
  doc/rev-list-options: document --first-parent changes merges format
  doc/diff-generate-patch: mention new --diff-merges option
  doc/git-log: describe new --diff-merges options
  diff-merges: add '--diff-merges=1' as synonym for 'first-parent'
  diff-merges: add old mnemonic counterparts to --diff-merges
  diff-merges: let new options enable diff without -p
  diff-merges: do not imply -p for new options
  diff-merges: implement new values for --diff-merges
  diff-merges: make -m/-c/--cc explicitly mutually exclusive
  diff-merges: refactor opt settings into separate functions
  diff-merges: get rid of now empty diff_merges_init_revs()
  diff-merges: group diff-merge flags next to each other inside 'rev_info'
  diff-merges: split 'ignore_merges' field
  diff-merges: fix -m to properly override -c/--cc
  t4013: add tests for -m failing to override -c/--cc
  t4013: support test_expect_failure through ':failure' magic
  diff-merges: revise revs->diff flag handling
  diff-merges: handle imply -p on -c/--cc logic for log.c
  ...

Documentation/diff-generate-patch.txt

@@ -81,9 +81,9 @@ Combined diff format
 Any diff-generating command can take the `-c` or `--cc` option to
 produce a 'combined diff' when showing a merge. This is the default
 format when showing merges with linkgit:git-diff[1] or
-linkgit:git-show[1]. Note also that you can give the `-m` option to any
-of these commands to force generation of diffs with individual parents
-of a merge.
+linkgit:git-show[1]. Note also that you can give suitable
+`--diff-merges` option to any of these commands to force generation of
+diffs in specific format.
 
 A "combined diff" format looks like this:
 

Documentation/diff-options.txt

@@ -33,6 +33,57 @@ endif::git-diff[]
 	show the patch by default, or to cancel the effect of `--patch`.
 endif::git-format-patch[]
 
+ifdef::git-log[]
+--diff-merges=(off|none|first-parent|1|separate|m|combined|c|dense-combined|cc)::
+--no-diff-merges::
+	Specify diff format to be used for merge commits. Default is
+	{diff-merges-default} unless `--first-parent` is in use, in which case
+	`first-parent` is the default.
++
+--diff-merges=(off|none):::
+--no-diff-merges:::
+	Disable output of diffs for merge commits. Useful to override
+	implied value.
++
+--diff-merges=first-parent:::
+--diff-merges=1:::
+	This option makes merge commits show the full diff with
+	respect to the first parent only.
++
+--diff-merges=separate:::
+--diff-merges=m:::
+-m:::
+	This makes merge commits show the full diff with respect to
+	each of the parents. Separate log entry and diff is generated
+	for each parent. `-m` doesn't produce any output without `-p`.
++
+--diff-merges=combined:::
+--diff-merges=c:::
+-c:::
+	With this option, diff output for a merge commit shows the
+	differences from each of the parents to the merge result
+	simultaneously instead of showing pairwise diff between a
+	parent and the result one at a time. Furthermore, it lists
+	only files which were modified from all parents. `-c` implies
+	`-p`.
++
+--diff-merges=dense-combined:::
+--diff-merges=cc:::
+--cc:::
+	With this option the output produced by
+	`--diff-merges=combined` is further compressed by omitting
+	uninteresting hunks whose contents in the parents have only
+	two variants and the merge result picks one of them without
+	modification.  `--cc` implies `-p`.
+
+--combined-all-paths::
+	This flag causes combined diffs (used for merge commits) to
+	list the name of the file from all parents.  It thus only has
+	effect when `--diff-merges=[dense-]combined` is in use, and
+	is likely only useful if filename changes are detected (i.e.
+	when either rename or copy detection have been requested).
+endif::git-log[]
+
 -U<n>::
 --unified=<n>::
 	Generate diffs with <n> lines of context instead of

Documentation/git-log.txt

@@ -107,47 +107,15 @@ DIFF FORMATTING
 By default, `git log` does not generate any diff output. The options
 below can be used to show the changes made by each commit.
 
-Note that unless one of `-c`, `--cc`, or `-m` is given, merge commits
-will never show a diff, even if a diff format like `--patch` is
-selected, nor will they match search options like `-S`. The exception is
-when `--first-parent` is in use, in which merges are treated like normal
-single-parent commits (this can be overridden by providing a
-combined-diff option or with `--no-diff-merges`).
-
--c::
-	With this option, diff output for a merge commit
-	shows the differences from each of the parents to the merge result
-	simultaneously instead of showing pairwise diff between a parent
-	and the result one at a time. Furthermore, it lists only files
-	which were modified from all parents.
-
---cc::
-	This flag implies the `-c` option and further compresses the
-	patch output by omitting uninteresting hunks whose contents in
-	the parents have only two variants and the merge result picks
-	one of them without modification.
-
---combined-all-paths::
-	This flag causes combined diffs (used for merge commits) to
-	list the name of the file from all parents.  It thus only has
-	effect when -c or --cc are specified, and is likely only
-	useful if filename changes are detected (i.e. when either
-	rename or copy detection have been requested).
-
--m::
-	This flag makes the merge commits show the full diff like
-	regular commits; for each merge parent, a separate log entry
-	and diff is generated. An exception is that only diff against
-	the first parent is shown when `--first-parent` option is given;
-	in that case, the output represents the changes the merge
-	brought _into_ the then-current branch.
-
---diff-merges=off::
---no-diff-merges::
-	Disable output of diffs for merge commits (default). Useful to
-	override `-m`, `-c`, or `--cc`.
+Note that unless one of `--diff-merges` variants (including short
+`-m`, `-c`, and `--cc` options) is explicitly given, merge commits
+will not show a diff, even if a diff format like `--patch` is
+selected, nor will they match search options like `-S`. The exception
+is when `--first-parent` is in use, in which case `first-parent` is
+the default format.
 
 :git-log: 1
+:diff-merges-default: `off`
 include::diff-options.txt[]
 
 include::diff-generate-patch.txt[]

Documentation/git-show.txt

@@ -45,10 +45,13 @@ include::pretty-options.txt[]
 include::pretty-formats.txt[]
 
 
-COMMON DIFF OPTIONS
--------------------
+DIFF FORMATTING
+---------------
+The options below can be used to change the way `git show` generates
+diff output.
 
 :git-log: 1
+:diff-merges-default: `dense-combined`
 include::diff-options.txt[]
 
 include::diff-generate-patch.txt[]

Documentation/rev-list-options.txt

@@ -130,6 +130,11 @@ parents) and `--max-parents=-1` (negative numbers denote no upper limit).
 	this option allows you to ignore the individual commits
 	brought in to your history by such a merge.
 
+ifdef::git-log[]
+	This option also changes default diff format for merge commits
+	to `first-parent`, see `--diff-merges=first-parent` for details.
+endif::git-log[]
+
 --not::
 	Reverses the meaning of the '{caret}' prefix (or lack thereof)
 	for all following revision specifiers, up to the next `--not`.