git-config: print error message if the config file cannot be read

Instead of simply exiting with 255, print an error message including
the reason why a config file specified through --file cannot be opened
or read.

The problem was noticed by Joey Hess, reported through
 http://bugs.debian.org/445208

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

builtin-config.c

@@ -175,7 +175,10 @@ int cmd_config(int argc, const char **argv, const char *prefix)
 		else if (!strcmp(argv[1], "--list") || !strcmp(argv[1], "-l")) {
 			if (argc != 2)
 				usage(git_config_set_usage);
-			return git_config(show_all_config);
+			if (git_config(show_all_config) < 0 && file && errno)
+				die("unable to read config file %s: %s", file,
+				    strerror(errno));
+			return 0;
 		}
 		else if (!strcmp(argv[1], "--global")) {
 			char *home = getenv("HOME");