format-patch: allow --range-diff to apply to a lone-patch

When submitting a revised version of a patch or series, it can be
helpful (to reviewers) to include a summary of changes since the
previous attempt in the form of a range-diff, typically in the cover
letter. However, it is occasionally useful, despite making for a noisy
read, to insert a range-diff into the commentary section of the lone
patch of a 1-patch series.

Therefore, extend "git format-patch --range-diff=<refspec>" to insert a
range-diff into the commentary section of a lone patch rather than
requiring a cover letter.

Implementation note: Generating a range-diff for insertion into the
commentary section of a patch which itself is currently being generated
requires invoking the diffing machinery recursively. However, the
machinery does not (presently) support this since it uses global state.
Consequently, we need to take care to stash away the state of the
in-progress operation while generating the range-diff, and restore it
after.

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

Documentation/git-format-patch.txt

@@ -241,7 +241,8 @@ feeding the result to `git send-email`.
 
 --range-diff=<previous>::
 	As a reviewer aid, insert a range-diff (see linkgit:git-range-diff[1])
-	into the cover letter showing the differences between the previous
+	into the cover letter, or as commentary of the lone patch of a
+	1-patch series, showing the differences between the previous
 	version of the patch series and the series currently being formatted.
 	`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

builtin/log.c

@@ -1576,7 +1576,7 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 			     N_("show changes against <rev> in cover letter or single patch"),
 			     parse_opt_object_name),
 		OPT_STRING(0, "range-diff", &rdiff_prev, N_("refspec"),
-			   N_("show changes against <refspec> in cover letter")),
+			   N_("show changes against <refspec> in cover letter or single patch")),
 		OPT_INTEGER(0, "creation-factor", &creation_factor,
 			    N_("percentage by which creation is weighted")),
 		OPT_END()
@@ -1817,8 +1817,8 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 		die(_("--creation-factor requires --range-diff"));
 
 	if (rdiff_prev) {
-		if (!cover_letter)
-			die(_("--range-diff requires --cover-letter"));
+		if (!cover_letter && total != 1)
+			die(_("--range-diff requires --cover-letter or single patch"));
 
 		infer_range_diff_ranges(&rdiff1, &rdiff2, rdiff_prev,
 					origin, list[0]);
@@ -1867,8 +1867,9 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 		print_signature(rev.diffopt.file);
 		total++;
 		start_number--;
-		/* interdiff in cover-letter; omit from patches */
+		/* interdiff/range-diff in cover-letter; omit from patches */
 		rev.idiff_oid1 = NULL;
+		rev.rdiff1 = NULL;
 	}
 	rev.add_signoff = do_signoff;
 

log-tree.c

@@ -16,6 +16,7 @@
 #include "line-log.h"
 #include "help.h"
 #include "interdiff.h"
+#include "range-diff.h"
 
 static struct decoration name_decoration = { "object names" };
 static int decoration_loaded;
@@ -751,6 +752,20 @@ void show_log(struct rev_info *opt)
 
 		memcpy(&diff_queued_diff, &dq, sizeof(diff_queued_diff));
 	}
+
+	if (cmit_fmt_is_mail(ctx.fmt) && opt->rdiff1) {
+		struct diff_queue_struct dq;
+
+		memcpy(&dq, &diff_queued_diff, sizeof(diff_queued_diff));
+		DIFF_QUEUE_CLEAR(&diff_queued_diff);
+
+		next_commentary_block(opt, NULL);
+		fprintf_ln(opt->diffopt.file, "%s", opt->rdiff_title);
+		show_range_diff(opt->rdiff1, opt->rdiff2,
+				opt->creation_factor, 1, &opt->diffopt);
+
+		memcpy(&diff_queued_diff, &dq, sizeof(diff_queued_diff));
+	}
 }
 
 int log_tree_diff_flush(struct rev_info *opt)