commit: More generous accepting of RFC-2822 footer lines.

'git commit -s' will insert a blank line before the Signed-off-by
line at the end of the message, unless this last line is a
Signed-off-by line itself.  Common use has other trailing lines
at the ends of commit text, in the style of RFC2822 headers.

Be more generous in considering lines to be part of this footer.
If the last paragraph of the commit message reasonably resembles
RFC-2822 formatted lines, don't insert that blank line.

The new Signed-off-by line is still only suppressed when the
author's existing Signed-off-by is the last line of the message.

Signed-off-by: David Brown <davidb@quicinc.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-commit.c

@@ -414,6 +414,47 @@ static void determine_author_info(void)
 	author_date = date;
 }
 
+static int ends_rfc2822_footer(struct strbuf *sb)
+{
+	int ch;
+	int hit = 0;
+	int i, j, k;
+	int len = sb->len;
+	int first = 1;
+	const char *buf = sb->buf;
+
+	for (i = len - 1; i > 0; i--) {
+		if (hit && buf[i] == '\n')
+			break;
+		hit = (buf[i] == '\n');
+	}
+
+	while (i < len - 1 && buf[i] == '\n')
+		i++;
+
+	for (; i < len; i = k) {
+		for (k = i; k < len && buf[k] != '\n'; k++)
+			; /* do nothing */
+		k++;
+
+		if ((buf[k] == ' ' || buf[k] == '\t') && !first)
+			continue;
+
+		first = 0;
+
+		for (j = 0; i + j < len; j++) {
+			ch = buf[i + j];
+			if (ch == ':')
+				break;
+			if (isalnum(ch) ||
+			    (ch == '-'))
+				continue;
+			return 0;
+		}
+	}
+	return 1;
+}
+
 static int prepare_to_commit(const char *index_file, const char *prefix,
 			     struct wt_status *s)
 {
@@ -489,7 +530,7 @@ static int prepare_to_commit(const char *index_file, const char *prefix,
 		for (i = sb.len - 1; i > 0 && sb.buf[i - 1] != '\n'; i--)
 			; /* do nothing */
 		if (prefixcmp(sb.buf + i, sob.buf)) {
-			if (prefixcmp(sb.buf + i, sign_off_header))
+			if (!ends_rfc2822_footer(&sb))
 				strbuf_addch(&sb, '\n');
 			strbuf_addbuf(&sb, &sob);
 		}

t/t7501-commit.sh

@@ -247,6 +247,47 @@ $existing" &&
 
 '
 
+test_expect_success 'signoff gap' '
+
+	echo 3 >positive &&
+	git add positive &&
+	alt="Alt-RFC-822-Header: Value" &&
+	git commit -s -m "welcome
+
+$alt" &&
+	git cat-file commit HEAD | sed -e "1,/^\$/d" > actual &&
+	(
+		echo welcome
+		echo
+		echo $alt
+		git var GIT_COMMITTER_IDENT |
+		sed -e "s/>.*/>/" -e "s/^/Signed-off-by: /"
+	) >expected &&
+	test_cmp expected actual
+'
+
+test_expect_success 'signoff gap 2' '
+
+	echo 4 >positive &&
+	git add positive &&
+	alt="fixed: 34" &&
+	git commit -s -m "welcome
+
+We have now
+$alt" &&
+	git cat-file commit HEAD | sed -e "1,/^\$/d" > actual &&
+	(
+		echo welcome
+		echo
+		echo We have now
+		echo $alt
+		echo
+		git var GIT_COMMITTER_IDENT |
+		sed -e "s/>.*/>/" -e "s/^/Signed-off-by: /"
+	) >expected &&
+	test_cmp expected actual
+'
+
 test_expect_success 'multiple -m' '
 
 	>negative &&