range-diff: introduce the convenience option --remerge-diff

Just like `git log`, now also `git range-diff` has that option as a
shortcut for the common operation that would otherwise require the quite
unwieldy (if theoretically "more correct") `--diff-mode=remerge` option.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

Documentation/git-range-diff.txt

@@ -11,6 +11,7 @@ SYNOPSIS
 'git range-diff' [--color=[<when>]] [--no-color] [<diff-options>]
 	[--no-dual-color] [--creation-factor=<factor>]
 	[--left-only | --right-only] [--diff-merges=<format>]
+	[--remerge-diff]
 	( <range1> <range2> | <rev1>...<rev2> | <base> <rev1> <rev2> )
 	[[--] <path>...]
 
@@ -92,6 +93,9 @@ have produced. In other words, if a merge commit is the result of a
 non-conflicting `git merge`, the `remerge` mode will represent it with an empty
 diff.
 
+--remerge-diff::
+	Convenience option, equivalent to `--diff-merges=remerge`.
+
 --[no-]notes[=<ref>]::
 	This flag is passed to the `git log` program
 	(see linkgit:git-log[1]) that generates the patches.

builtin/range-diff.c

@@ -34,6 +34,8 @@ int cmd_range_diff(int argc, const char **argv, const char *prefix)
 				  PARSE_OPT_OPTARG),
 		OPT_PASSTHRU_ARGV(0, "diff-merges", &diff_merges_arg,
 				  N_("style"), N_("passed to 'git log'"), 0),
+		OPT_PASSTHRU_ARGV(0, "remerge-diff", &diff_merges_arg, NULL,
+				  N_("passed to 'git log'"), PARSE_OPT_NOARG),
 		OPT_BOOL(0, "left-only", &left_only,
 			 N_("only emit output related to the first range")),
 		OPT_BOOL(0, "right-only", &right_only,