format-patch: add --interdiff option to embed diff in cover letter

When submitting a revised version of a patch series, it can be helpful
(to reviewers) to include a summary of changes since the previous
attempt in the form of an interdiff, however, doing so involves manually
copy/pasting the diff into the cover letter.

Add an --interdiff option to automate this process. The argument to
--interdiff specifies the tip of the previous attempt against which to
generate the interdiff. For example:

    git format-patch --cover-letter --interdiff=v1 -3 v2

The previous attempt and the patch series being formatted must share a
common base.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 22 insertions, 2 deletions

diff --git a/builtin/log.c b/builtin/log.c
index 873aabcf40..1020b78477 100644
--- a/builtin/log.c
+++ b/builtin/log.c
@@ -30,6 +30,7 @@
 #include "gpg-interface.h"
 #include "progress.h"
 #include "commit-slab.h"
+#include "interdiff.h"
 
 #define MAIL_DEFAULT_WRAP 72
 
@@ -1082,6 +1083,11 @@ static void make_cover_letter(struct rev_info *rev, int use_stdout,
 	/* We can only do diffstat with a unique reference point */
 	if (origin)
 		show_diffstat(rev, origin, head);
+
+	if (rev->idiff_oid1) {
+		fprintf_ln(rev->diffopt.file, "%s", _("Interdiff:"));
+		show_interdiff(rev);
+	}
 }
 
 static const char *clean_message_id(const char *msg_id)
@@ -1448,6 +1454,7 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 	struct base_tree_info bases;
 	int show_progress = 0;
 	struct progress *progress = NULL;
+	struct oid_array idiff_prev = OID_ARRAY_INIT;
 
 	const struct option builtin_format_patch_options[] = {
 		{ OPTION_CALLBACK, 'n', "numbered", &numbered, NULL,
@@ -1521,6 +1528,9 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 		OPT__QUIET(&quiet, N_("don't print the patch filenames")),
 		OPT_BOOL(0, "progress", &show_progress,
 			 N_("show progress while generating patches")),
+		OPT_CALLBACK(0, "interdiff", &idiff_prev, N_("rev"),
+			     N_("show changes against <rev> in cover letter"),
+			     parse_opt_object_name),
 		OPT_END()
 	};
 
@@ -1706,7 +1716,7 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 		if (rev.pending.nr == 2) {
 			struct object_array_entry *o = rev.pending.objects;
 			if (oidcmp(&o[0].item->oid, &o[1].item->oid) == 0)
-				return 0;
+				goto done;
 		}
 		get_patch_ids(&rev, &ids);
 	}
@@ -1730,7 +1740,7 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 	}
 	if (nr == 0)
 		/* nothing to do */
-		return 0;
+		goto done;
 	total = nr;
 	if (cover_letter == -1) {
 		if (config_cover_letter == COVER_AUTO)
@@ -1743,6 +1753,13 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 	if (numbered)
 		rev.total = total + start_number - 1;
 
+	if (idiff_prev.nr) {
+		if (!cover_letter)
+			die(_("--interdiff requires --cover-letter"));
+		rev.idiff_oid1 = &idiff_prev.oid[idiff_prev.nr - 1];
+		rev.idiff_oid2 = get_commit_tree_oid(list[0]);
+	}
+
 	if (!signature) {
 		; /* --no-signature inhibits all signatures */
 	} else if (signature && signature != git_version_string) {
@@ -1860,6 +1877,9 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 	string_list_clear(&extra_hdr, 0);
 	if (ignore_if_in_upstream)
 		free_patch_ids(&ids);
+
+done:
+	oid_array_clear(&idiff_prev);
 	return 0;
 }