Support for extracting configuration from different files

Add $GIT_CONFIG environment variable whose content is used instead
of .git/config if set. Also add $GIT_CONFIG_LOCAL as a
forward-compatibility cue for whenever we will finally come to support]
global configuration files (properly).

Signed-off-by: Petr Baudis <pasky@suse.cz>
Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/git-repo-config.txt

@@ -73,6 +73,18 @@ OPTIONS
 	List all variables set in .git/config.
 
 
+ENVIRONMENT
+-----------
+
+GIT_CONFIG::
+	Take the configuration from the given file instead of .git/config.
+
+GIT_CONFIG_LOCAL::
+	Currently the same as $GIT_CONFIG; when Git will support global
+	configuration files, this will cause it to take the configuration
+	from the global configuration file in addition to the given file.
+
+
 EXAMPLE
 -------
 

config.c

@@ -317,7 +317,17 @@ int git_config_from_file(config_fn_t fn, const char *filename)
 
 int git_config(config_fn_t fn)
 {
-	return git_config_from_file(fn, git_path("config"));
+	const char *filename = git_path("config");
+	/* Forward-compatibility cue: $GIT_CONFIG makes git read _only_
+	 * the given config file, $GIT_CONFIG_LOCAL will make it process
+	 * it in addition to the global config file, the same way it would
+	 * the per-repository config file otherwise. */
+	if (getenv("GIT_CONFIG")) {
+		filename = getenv("GIT_CONFIG");
+	} else if (getenv("GIT_CONFIG_LOCAL")) {
+		filename = getenv("GIT_CONFIG_LOCAL");
+	}
+	return git_config_from_file(fn, filename);
 }
 
 /*