[RFC PATCH 3/5] format-patch: extend --range-diff to accept revision range

[Eric Sunshine <sunshine@sunshineco.com>]

When submitting a revised a patch series, the --range-diff option embeds
a range-diff in the cover letter showing changes since the previous
version of the patch series. The argument to --range-diff is a simple
revision naming the tip of the previous series, which works fine if the
previous and current versions of the patch series share a common base.

However, it fails if the revision ranges of the old and new versions of
the series are disjoint. To address this shortcoming, extend
--range-diff to also accept an explicit revision range for the previous
series. For example:

    git format-patch --cover-letter --range-diff=v1~3..v1 -3 v2

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
---
 Documentation/git-format-patch.txt |  8 +++++---
 builtin/log.c                      | 16 +++++++++++++---
 t/t7910-branch-diff.sh             |  1 +
 3 files changed, 19 insertions(+), 6 deletions(-)

diff --git a/Documentation/git-format-patch.txt b/Documentation/git-format-patch.txt
index f4c70e6b64..25026ae26e 100644
--- a/Documentation/git-format-patch.txt
+++ b/Documentation/git-format-patch.txt
@@ -233,10 +233,12 @@ feeding the result to `git send-email`.
 	As a reviewer aid, insert a range-diff (see linkgit:git-branch-diff[1])
 	into the cover letter showing the differences between the previous
 	version of the patch series and the series currently being formatted.
-	`previous` is a single revision naming the tip of the previous
-	series which shares a common base with the series being formatted (for
+	`previous` can be a single revision naming the tip of the previous
+	series if it shares a common base with the series being formatted (for
 	example `git format-patch --cover-letter --range-diff=feature/v1 -3
-	feature/v2`).
+	feature/v2`), or a revision range if the two versions of the series are
+	disjoint (for example `git format-patch --cover-letter
+	--range-diff=feature/v1~3..feature/v1 -3 feature/v2`).
 
 --notes[=<ref>]::
 	Append the notes (see linkgit:git-notes[1]) for the commit
diff --git a/builtin/log.c b/builtin/log.c
index 460c31a293..e38cf06050 100644
--- a/builtin/log.c
+++ b/builtin/log.c
@@ -995,10 +995,20 @@ static char *find_branch_name(struct rev_info *rev)
 
 static void infer_diff_ranges(struct argv_array *args,
 			      const char *prev,
+			      struct commit *origin,
 			      struct commit *head)
 {
-	argv_array_pushf(args, "%s...%s", prev,
-			 oid_to_hex(&head->object.oid));
+	if (strstr(prev, "..")) {
+		if (!origin)
+			die(_("failed to infer range-diff ranges"));
+		argv_array_push(args, prev);
+		argv_array_pushf(args, "%s..%s",
+				 oid_to_hex(&origin->object.oid),
+				 oid_to_hex(&head->object.oid));
+	} else {
+		argv_array_pushf(args, "%s...%s", prev,
+				 oid_to_hex(&head->object.oid));
+	}
 }
 
 static int get_range_diff(struct strbuf *sb,
@@ -1059,7 +1069,7 @@ static void make_cover_letter(struct rev_info *rev, int use_stdout,
 	/* might die from bad user input so try before creating cover letter */
 	if (range_diff) {
 		struct argv_array ranges = ARGV_ARRAY_INIT;
-		infer_diff_ranges(&ranges, range_diff, head);
+		infer_diff_ranges(&ranges, range_diff, origin, head);
 		if (get_range_diff(&diff, &ranges))
 			die(_("failed to generate range-diff"));
 		argv_array_clear(&ranges);
diff --git a/t/t7910-branch-diff.sh b/t/t7910-branch-diff.sh
index edbd69b6f8..c0e8668ed9 100755
--- a/t/t7910-branch-diff.sh
+++ b/t/t7910-branch-diff.sh
@@ -155,5 +155,6 @@ format_patch () {
 }
 
 format_patch 'B...C' topic
+format_patch 'A..B A..C' master..topic
 
 test_done
-- 
2.17.1.1235.ge295dfb56e