format-patch: preserve subject newlines with -k

In older versions of git, we used rfc822 header folding to
indicate that the original subject line had multiple lines
in it.  But since a1f6baa (format-patch: wrap long header
lines, 2011-02-23), we now use header folding whenever there
is a long line.

This means that "git am" cannot trust header folding as a
sign from format-patch that newlines should be preserved.
Instead, format-patch needs to signal more explicitly that
the newlines are significant.  This patch does so by
rfc2047-encoding the newlines in the subject line. No
changes are needed on the "git am" end; it already decodes
the newlines properly.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/log.c

@@ -1131,6 +1131,7 @@ int cmd_format_patch(int argc, const char **argv, const char *prefix)
 		die ("-n and -k are mutually exclusive.");
 	if (keep_subject && subject_prefix)
 		die ("--subject-prefix and -k are mutually exclusive.");
+	rev.preserve_subject = keep_subject;
 
 	argc = setup_revisions(argc, argv, &rev, &s_r_opt);
 	if (argc > 1)

commit.h

@@ -74,6 +74,7 @@ struct pretty_print_context
 	int abbrev;
 	const char *subject;
 	const char *after_subject;
+	int preserve_subject;
 	enum date_mode date_mode;
 	int need_8bit_cte;
 	int show_notes;

log-tree.c

@@ -504,6 +504,7 @@ void show_log(struct rev_info *opt)
 	ctx.date_mode = opt->date_mode;
 	ctx.abbrev = opt->diffopt.abbrev;
 	ctx.after_subject = extra_headers;
+	ctx.preserve_subject = opt->preserve_subject;
 	ctx.reflog_info = opt->reflog_info;
 	ctx.fmt = opt->commit_format;
 	pretty_print_commit(&ctx, commit, &msgbuf);

pretty.c

@@ -1131,7 +1131,8 @@ void pp_title_line(const struct pretty_print_context *pp,
 	struct strbuf title;
 
 	strbuf_init(&title, 80);
-	*msg_p = format_subject(&title, *msg_p, " ");
+	*msg_p = format_subject(&title, *msg_p,
+				pp->preserve_subject ? "\n" : " ");
 
 	strbuf_grow(sb, title.len + 1024);
 	if (pp->subject) {

revision.h

@@ -90,7 +90,8 @@ struct rev_info {
 			abbrev_commit:1,
 			use_terminator:1,
 			missing_newline:1,
-			date_mode_explicit:1;
+			date_mode_explicit:1,
+			preserve_subject:1;
 	unsigned int	disable_stdin:1;
 
 	enum date_mode date_mode;

t/t4152-am-subjects.sh

@@ -70,7 +70,7 @@ test_expect_success 'multiline subject unwrapped (format-patch -k | am)' '
 	check_subject multiline-k
 '
 echo "$MULTILINE_SUBJECT" >expect
-test_expect_failure 'multiline subject preserved (format-patch -k | am -k)' '
+test_expect_success 'multiline subject preserved (format-patch -k | am -k)' '
 	check_subject multiline-k -k
 '