git-config: handle --file option with relative pathname properly

When calling git-config not from the top level directory of a repository,
it changes directory before trying to open the config file specified
through the --file option, which then fails if the config file was
specified by a relative pathname.  This patch adjusts the pathname to
the config file if applicable.

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

Signed-off-by: Gerrit Pape <pape@smarden.org>
Signed-off-by: Lars Hjemli <hjemli@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

builtin-config.c

@@ -165,7 +165,7 @@ int cmd_config(int argc, const char **argv, const char *prefix)
 {
 	int nongit = 0;
 	char* value;
-	setup_git_directory_gently(&nongit);
+	const char *file = setup_git_directory_gently(&nongit);
 
 	while (1 < argc) {
 		if (!strcmp(argv[1], "--int"))
@@ -192,7 +192,12 @@ int cmd_config(int argc, const char **argv, const char *prefix)
 		else if (!strcmp(argv[1], "--file") || !strcmp(argv[1], "-f")) {
 			if (argc < 3)
 				usage(git_config_set_usage);
-			setenv(CONFIG_ENVIRONMENT, argv[2], 1);
+			if (!is_absolute_path(argv[2]) && file)
+				file = prefix_filename(file, strlen(file),
+						       argv[2]);
+			else
+				file = argv[2];
+			setenv(CONFIG_ENVIRONMENT, file, 1);
 			argc--;
 			argv++;
 		}