Fix renaming branch without config file

Make git_config_rename_section return success if no config file
exists.  Otherwise, renaming a branch would abort, leaving the
repository in an inconsistent state.

[jc: test]

Signed-off-by: Geert Bosch <bosch@gnat.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>

config.c

@@ -916,8 +916,8 @@ int git_config_rename_section(const char *old_name, const char *new_name)
 	}
 
 	if (!(config_file = fopen(config_filename, "rb"))) {
-		ret = error("Could not open config file!");
-		goto out;
+		/* no config file means nothing to rename, no error */
+		goto unlock_and_out;
 	}
 
 	while (fgets(buf, sizeof(buf), config_file)) {
@@ -951,6 +951,7 @@ int git_config_rename_section(const char *old_name, const char *new_name)
 		}
 	}
 	fclose(config_file);
+ unlock_and_out:
 	if (close(out_fd) || commit_lock_file(lock) < 0)
 			ret = error("Cannot commit config file!");
  out:

t/t3200-branch.sh

@@ -83,6 +83,15 @@ test_expect_failure \
          git-branch r &&
          git-branch -m q r/q'
 
+mv .git/config .git/config-saved
+
+test_expect_success 'git branch -m q Q without config should succeed' '
+	git-branch -m q Q &&
+	git-branch -m Q q
+'
+
+mv .git/config-saved .git/config
+
 git-config branch.s/s.dummy Hello
 
 test_expect_success \