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>

1 files changed, 15 insertions, 0 deletions

diff --git a/log-tree.c b/log-tree.c
index e21c3d959f..cec983a461 100644
--- a/log-tree.c
+++ b/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)