config: fix leaking comment character config

When the comment line character has been specified multiple times in the
configuration, then `git_default_core_config()` will cause a memory leak
because it unconditionally copies the string into `comment_line_str`
without free'ing the previous value. In fact, it can't easily free the
value in the first place because it may contain a string constant.

Refactor the code such that we track allocated comment character strings
via a separate non-constant variable `comment_line_str_to_free`. Adapt
sites that set `comment_line_str` to set both and free the old value
that was stored in `comment_line_str_to_free`.

This memory leak is being hit in t3404. As there are still other memory
leaks in that file we cannot yet mark it as passing with leak checking
enabled.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/commit.c

@@ -684,7 +684,9 @@ static void adjust_comment_line_char(const struct strbuf *sb)
 	const char *p;
 
 	if (!memchr(sb->buf, candidates[0], sb->len)) {
-		comment_line_str = xstrfmt("%c", candidates[0]);
+		free(comment_line_str_to_free);
+		comment_line_str = comment_line_str_to_free =
+			xstrfmt("%c", candidates[0]);
 		return;
 	}
 
@@ -705,7 +707,8 @@ static void adjust_comment_line_char(const struct strbuf *sb)
 	if (!*p)
 		die(_("unable to select a comment character that is not used\n"
 		      "in the current commit message"));
-	comment_line_str = xstrfmt("%c", *p);
+	free(comment_line_str_to_free);
+	comment_line_str = comment_line_str_to_free = xstrfmt("%c", *p);
 }
 
 static void prepare_amend_commit(struct commit *commit, struct strbuf *sb,

config.c

@@ -1596,7 +1596,8 @@ static int git_default_core_config(const char *var, const char *value,
 		else if (value[0]) {
 			if (strchr(value, '\n'))
 				return error(_("%s cannot contain newline"), var);
-			comment_line_str = xstrdup(value);
+			comment_line_str = value;
+			FREE_AND_NULL(comment_line_str_to_free);
 			auto_comment_line_char = 0;
 		} else
 			return error(_("%s must have at least one character"), var);

environment.c

@@ -114,6 +114,7 @@ int protect_ntfs = PROTECT_NTFS_DEFAULT;
  * that is subject to stripspace.
  */
 const char *comment_line_str = "#";
+char *comment_line_str_to_free;
 int auto_comment_line_char;
 
 /* Parallel index stat data preload? */

environment.h

@@ -9,6 +9,7 @@ struct strvec;
  * that is subject to stripspace.
  */
 extern const char *comment_line_str;
+extern char *comment_line_str_to_free;
 extern int auto_comment_line_char;
 
 /*