config: teach git_config_rename_section a file argument

The other config-writing functions (git_config_set and
git_config_set_multivar) each have an -"in_file" version to
write a specific file. Let's add one for rename_section,
with the eventual goal of moving away from the magic
config_exclusive_filename global.

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

cache.h

@@ -1128,6 +1128,7 @@ extern int git_config_parse_key(const char *, char **, int *);
 extern int git_config_set_multivar(const char *, const char *, const char *, int);
 extern int git_config_set_multivar_in_file(const char *, const char *, const char *, const char *, int);
 extern int git_config_rename_section(const char *, const char *);
+extern int git_config_rename_section_in_file(const char *, const char *, const char *);
 extern const char *git_etc_gitconfig(void);
 extern int check_repository_format_version(const char *var, const char *value, void *cb);
 extern int git_env_bool(const char *, int);

config.c

@@ -1470,19 +1470,19 @@ static int section_name_match (const char *buf, const char *name)
 }
 
 /* if new_name == NULL, the section is removed instead */
-int git_config_rename_section(const char *old_name, const char *new_name)
+int git_config_rename_section_in_file(const char *config_filename,
+				      const char *old_name, const char *new_name)
 {
 	int ret = 0, remove = 0;
-	char *config_filename;
+	char *filename_buf = NULL;
 	struct lock_file *lock = xcalloc(sizeof(struct lock_file), 1);
 	int out_fd;
 	char buf[1024];
 	FILE *config_file;
 
-	if (config_exclusive_filename)
-		config_filename = xstrdup(config_exclusive_filename);
-	else
-		config_filename = git_pathdup("config");
+	if (!config_filename)
+		config_filename = filename_buf = git_pathdup("config");
+
 	out_fd = hold_lock_file_for_update(lock, config_filename, 0);
 	if (out_fd < 0) {
 		ret = error("could not lock config file %s", config_filename);
@@ -1546,10 +1546,16 @@ int git_config_rename_section(const char *old_name, const char *new_name)
 	if (commit_lock_file(lock) < 0)
 		ret = error("could not commit config file %s", config_filename);
 out:
-	free(config_filename);
+	free(filename_buf);
 	return ret;
 }
 
+int git_config_rename_section(const char *old_name, const char *new_name)
+{
+	return git_config_rename_section_in_file(config_exclusive_filename,
+						 old_name, new_name);
+}
+
 /*
  * Call this to report error for your variable that should not
  * get a boolean value (i.e. "[my] var" means "true").