Merge branch 'ah/rebase-merges-config'

Streamline --rebase-merges command line option handling and
introduce rebase.merges configuration variable.

* ah/rebase-merges-config:
  rebase: add a config option for --rebase-merges
  rebase: deprecate --rebase-merges=""
  rebase: add documentation and test for --no-rebase-merges

Documentation/config/rebase.txt

@@ -67,3 +67,13 @@ rebase.rescheduleFailedExec::
 
 rebase.forkPoint::
 	If set to false set `--no-fork-point` option by default.
+
+rebase.rebaseMerges::
+	Whether and how to set the `--rebase-merges` option by default. Can
+	be `rebase-cousins`, `no-rebase-cousins`, or a boolean. Setting to
+	true or to `no-rebase-cousins` is equivalent to
+	`--rebase-merges=no-rebase-cousins`, setting to `rebase-cousins` is
+	equivalent to `--rebase-merges=rebase-cousins`, and setting to false is
+	equivalent to `--no-rebase-merges`. Passing `--rebase-merges` on the
+	command line, with or without an argument, overrides any
+	`rebase.rebaseMerges` configuration.

Documentation/git-rebase.txt

@@ -529,20 +529,25 @@ See also INCOMPATIBLE OPTIONS below.
 
 -r::
 --rebase-merges[=(rebase-cousins|no-rebase-cousins)]::
+--no-rebase-merges::
 	By default, a rebase will simply drop merge commits from the todo
 	list, and put the rebased commits into a single, linear branch.
 	With `--rebase-merges`, the rebase will instead try to preserve
 	the branching structure within the commits that are to be rebased,
 	by recreating the merge commits. Any resolved merge conflicts or
 	manual amendments in these merge commits will have to be
-	resolved/re-applied manually.
+	resolved/re-applied manually. `--no-rebase-merges` can be used to
+	countermand both the `rebase.rebaseMerges` config option and a previous
+	`--rebase-merges`.
 +
-By default, or when `no-rebase-cousins` was specified, commits which do not
-have `<upstream>` as direct ancestor will keep their original branch point,
-i.e. commits that would be excluded by linkgit:git-log[1]'s
-`--ancestry-path` option will keep their original ancestry by default. If
-the `rebase-cousins` mode is turned on, such commits are instead rebased
-onto `<upstream>` (or `<onto>`, if specified).
+When rebasing merges, there are two modes: `rebase-cousins` and
+`no-rebase-cousins`. If the mode is not specified, it defaults to
+`no-rebase-cousins`. In `no-rebase-cousins` mode, commits which do not have
+`<upstream>` as direct ancestor will keep their original branch point, i.e.
+commits that would be excluded by linkgit:git-log[1]'s `--ancestry-path`
+option will keep their original ancestry by default. In `rebase-cousins` mode,
+such commits are instead rebased onto `<upstream>` (or `<onto>`, if
+specified).
 +
 It is currently only possible to recreate the merge commits using the
 `ort` merge strategy; different merge strategies can be used only via