Merge branch 'sb/diff-orderfile-config'

Allow "git diff -O<file>" to be configured with a new configuration
variable.

* sb/diff-orderfile-config:
  diff: add diff.orderfile configuration variable
  diff: let "git diff -O" read orderfile from any file and fail properly
  t4056: add new tests for "git diff -O"

Documentation/diff-config.txt

@@ -98,6 +98,11 @@ diff.mnemonicprefix::
 diff.noprefix::
 	If set, 'git diff' does not show any source or destination prefix.
 
+diff.orderfile::
+	File indicating how to order files within a diff, using
+	one shell glob pattern per line.
+	Can be overridden by the '-O' option to linkgit:git-diff[1].
+
 diff.renameLimit::
 	The number of files to consider when performing the copy/rename
 	detection; equivalent to the 'git diff' option '-l'.

Documentation/diff-options.txt

@@ -432,6 +432,9 @@ endif::git-format-patch[]
 -O<orderfile>::
 	Output the patch in the order specified in the
 	<orderfile>, which has one shell glob pattern per line.
+	This overrides the `diff.orderfile` configuration variable
+	(see linkgit:git-config[1]).  To cancel `diff.orderfile`,
+	use `-O/dev/null`.
 
 ifndef::git-format-patch[]
 -R::