format-patch: extend --range-diff to accept revision range

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>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-format-patch.txt

@@ -243,10 +243,12 @@ feeding the result to `git send-email`.
 	As a reviewer aid, insert a range-diff (see linkgit:git-range-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

builtin/log.c

@@ -1448,12 +1448,21 @@ static const char *diff_title(struct strbuf *sb, int reroll_count,
 static void infer_range_diff_ranges(struct strbuf *r1,
 				    struct strbuf *r2,
 				    const char *prev,
+				    struct commit *origin,
 				    struct commit *head)
 {
 	const char *head_oid = oid_to_hex(&head->object.oid);
 
-	strbuf_addf(r1, "%s..%s", head_oid, prev);
-	strbuf_addf(r2, "%s..%s", prev, head_oid);
+	if (!strstr(prev, "..")) {
+		strbuf_addf(r1, "%s..%s", head_oid, prev);
+		strbuf_addf(r2, "%s..%s", prev, head_oid);
+	} else if (!origin) {
+		die(_("failed to infer range-diff ranges"));
+	} else {
+		strbuf_addstr(r1, prev);
+		strbuf_addf(r2, "%s..%s",
+			    oid_to_hex(&origin->object.oid), head_oid);
+	}
 }
 
 int cmd_format_patch(int argc, const char **argv, const char *prefix)
@@ -1802,7 +1811,8 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 		if (!cover_letter)
 			die(_("--range-diff requires --cover-letter"));
 
-		infer_range_diff_ranges(&rdiff1, &rdiff2, rdiff_prev, list[0]);
+		infer_range_diff_ranges(&rdiff1, &rdiff2, rdiff_prev,
+					origin, list[0]);
 		rev.rdiff1 = rdiff1.buf;
 		rev.rdiff2 = rdiff2.buf;
 		rev.creation_factor = RANGE_DIFF_CREATION_FACTOR_DEFAULT;

t/t3206-range-diff.sh

@@ -142,7 +142,7 @@ test_expect_success 'changed message' '
 	test_cmp expected actual
 '
 
-for prev in topic
+for prev in topic master..topic
 do
 	test_expect_success "format-patch --range-diff=$prev" '
 		git format-patch --stdout --cover-letter --range-diff=$prev \