[PATCH 13/14] format-patch: add --creation-factor tweak for --range-diff

[Eric Sunshine <sunshine@sunshineco.com>]

When generating a range-diff, matching up commits between two version of
a patch series involves heuristics, thus may give unexpected results.
git-range-diff allows tweaking the heuristic via --creation-factor.
Follow suit by accepting --creation-factor in combination with
--range-diff when generating a range-diff for a cover-letter.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
---
 Documentation/git-format-patch.txt |  8 +++++++-
 builtin/log.c                      | 10 +++++++++-
 2 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/Documentation/git-format-patch.txt b/Documentation/git-format-patch.txt
index 425145f536..9b2e172159 100644
--- a/Documentation/git-format-patch.txt
+++ b/Documentation/git-format-patch.txt
@@ -24,7 +24,7 @@ SYNOPSIS
 		   [--to=<email>] [--cc=<email>]
 		   [--[no-]cover-letter] [--quiet] [--notes[=<ref>]]
 		   [--interdiff=<previous>]
-		   [--range-diff=<previous>]
+		   [--range-diff=<previous> [--creation-factor=<percent>]]
 		   [--progress]
 		   [<common diff options>]
 		   [ <since> | <revision range> ]
@@ -250,6 +250,12 @@ feeding the result to `git send-email`.
 	disjoint (for example `git format-patch --cover-letter
 	--range-diff=feature/v1~3..feature/v1 -3 feature/v2`).
 
+--creation-factor=<percent>::
+	Used with `--range-diff`, tweak the heuristic which matches up commits
+	between the previous and current series of patches by adjusting the
+	creation/deletion cost fudge factor. See linkgit:git-range-diff[1])
+	for details.
+
 --notes[=<ref>]::
 	Append the notes (see linkgit:git-notes[1]) for the commit
 	after the three-dash line.
diff --git a/builtin/log.c b/builtin/log.c
index fdb2180d7e..10c3801ceb 100644
--- a/builtin/log.c
+++ b/builtin/log.c
@@ -1497,6 +1497,7 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 	struct strbuf rdiff1 = STRBUF_INIT;
 	struct strbuf rdiff2 = STRBUF_INIT;
 	struct strbuf rdiff_title = STRBUF_INIT;
+	int creation_factor = -1;
 
 	const struct option builtin_format_patch_options[] = {
 		{ OPTION_CALLBACK, 'n', "numbered", &numbered, NULL,
@@ -1575,6 +1576,8 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 			     parse_opt_object_name),
 		OPT_STRING(0, "range-diff", &rdiff_prev, N_("refspec"),
 			   N_("show changes against <refspec> in cover letter")),
+		OPT_INTEGER(0, "creation-factor", &creation_factor,
+			    N_("percentage by which creation is weighted")),
 		OPT_END()
 	};
 
@@ -1807,6 +1810,11 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 					     _("Interdiff against v%d:"));
 	}
 
+	if (creation_factor < 0)
+		creation_factor = RANGE_DIFF_CREATION_FACTOR_DEFAULT;
+	else if (!rdiff_prev)
+		die(_("--creation-factor requires --range-diff"));
+
 	if (rdiff_prev) {
 		if (!cover_letter)
 			die(_("--range-diff requires --cover-letter"));
@@ -1815,7 +1823,7 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 					origin, list[0]);
 		rev.rdiff1 = rdiff1.buf;
 		rev.rdiff2 = rdiff2.buf;
-		rev.creation_factor = RANGE_DIFF_CREATION_FACTOR_DEFAULT;
+		rev.creation_factor = creation_factor;
 		rev.rdiff_title = diff_title(&rdiff_title, reroll_count,
 					     _("Range-diff:"),
 					     _("Range-diff against v%d:"));
-- 
2.18.0.345.g5c9ce644c3