commit: allow core.commentChar=auto for character auto selection

When core.commentChar is "auto", the comment char starts with '#' as
in default but if it's already in the prepared message, find another
char in a small subset. This should stop surprises because git strips
some lines unexpectedly.

Note that git is not smart enough to recognize '#' as the comment char
in custom templates and convert it if the final comment char is
different. It thinks '#' lines in custom templates as part of the
commit message. So don't use this with custom templates.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -544,6 +544,9 @@ core.commentchar::
 	messages consider a line that begins with this character
 	commented, and removes them after the editor returns
 	(default '#').
++
+If set to "auto", `git-commit` would select a character that is not
+the beginning character of any line in existing commit messages.
 
 sequence.editor::
 	Text editor used by `git rebase -i` for editing the rebase instruction file.

builtin/commit.c

@@ -594,6 +594,36 @@ static char *cut_ident_timestamp_part(char *string)
 	return ket;
 }
 
+static void adjust_comment_line_char(const struct strbuf *sb)
+{
+	char candidates[] = "#;@!$%^&|:";
+	char *candidate;
+	const char *p;
+
+	comment_line_char = candidates[0];
+	if (!memchr(sb->buf, comment_line_char, sb->len))
+		return;
+
+	p = sb->buf;
+	candidate = strchr(candidates, *p);
+	if (candidate)
+		*candidate = ' ';
+	for (p = sb->buf; *p; p++) {
+		if ((p[0] == '\n' || p[0] == '\r') && p[1]) {
+			candidate = strchr(candidates, p[1]);
+			if (candidate)
+				*candidate = ' ';
+		}
+	}
+
+	for (p = candidates; *p == ' '; p++)
+		;
+	if (!*p)
+		die(_("unable to select a comment character that is not used\n"
+		      "in the current commit message"));
+	comment_line_char = *p;
+}
+
 static int prepare_to_commit(const char *index_file, const char *prefix,
 			     struct commit *current_head,
 			     struct wt_status *s,
@@ -748,6 +778,8 @@ static int prepare_to_commit(const char *index_file, const char *prefix,
 	if (fwrite(sb.buf, 1, sb.len, s->fp) < sb.len)
 		die_errno(_("could not write commit template"));
 
+	if (auto_comment_line_char)
+		adjust_comment_line_char(&sb);
 	strbuf_release(&sb);
 
 	/* This checks if committer ident is explicitly given */

cache.h

@@ -602,6 +602,7 @@ extern int precomposed_unicode;
  * that is subject to stripspace.
  */
 extern char comment_line_char;
+extern int auto_comment_line_char;
 
 enum branch_track {
 	BRANCH_TRACK_UNSPECIFIED = -1,

config.c

@@ -828,8 +828,11 @@ static int git_default_core_config(const char *var, const char *value)
 		int ret = git_config_string(&comment, var, value);
 		if (ret)
 			return ret;
+		else if (!strcasecmp(comment, "auto"))
+			auto_comment_line_char = 1;
 		else if (comment[0] && !comment[1]) {
 			comment_line_char = comment[0];
+			auto_comment_line_char = 0;
 		} else
 			return error("core.commentChar should only be one character");
 		return 0;

environment.c

@@ -69,6 +69,7 @@ unsigned long pack_size_limit_cfg;
  * that is subject to stripspace.
  */
 char comment_line_char = '#';
+int auto_comment_line_char;
 
 /* Parallel index stat data preload? */
 int core_preload_index = 0;

t/t7502-commit.sh

@@ -563,4 +563,30 @@ test_expect_success 'commit --status with custom comment character' '
 	test_i18ngrep "^; Changes to be committed:" .git/COMMIT_EDITMSG
 '
 
+test_expect_success 'switch core.commentchar' '
+	test_commit "#foo" foo &&
+	GIT_EDITOR=.git/FAKE_EDITOR git -c core.commentChar=auto commit --amend &&
+	test_i18ngrep "^; Changes to be committed:" .git/COMMIT_EDITMSG
+'
+
+test_expect_success 'switch core.commentchar but out of options' '
+	cat >text <<\EOF &&
+# 1
+; 2
+@ 3
+! 4
+$ 5
+% 6
+^ 7
+& 8
+| 9
+: 10
+EOF
+	git commit --amend -F text &&
+	(
+		test_set_editor .git/FAKE_EDITOR &&
+		test_must_fail git -c core.commentChar=auto commit --amend
+	)
+'
+
 test_done