Fix setting config variables with an alternative GIT_CONFIG

When setting a config variable, git_config_set() ignored the variables
GIT_CONFIG and GIT_CONFIG_LOCAL. Now, when GIT_CONFIG_LOCAL is set, it
will write to that file. If not, GIT_CONFIG is checked, and only as a
fallback, the change is written to $GIT_DIR/config.

Add a test for it, and also future-proof the test for the upcoming
$HOME/.gitconfig support.

Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>

config.c

@@ -500,10 +500,19 @@ int git_config_set_multivar(const char* key, const char* value,
 	int i, dot;
 	int fd = -1, in_fd;
 	int ret;
-	char* config_filename = strdup(git_path("config"));
-	char* lock_file = strdup(git_path("config.lock"));
+	char* config_filename;
+	char* lock_file;
 	const char* last_dot = strrchr(key, '.');
 
+	config_filename = getenv("GIT_CONFIG");
+	if (!config_filename) {
+		config_filename = getenv("GIT_CONFIG_LOCAL");
+		if (!config_filename)
+			config_filename  = git_path("config");
+	}
+	config_filename = strdup(config_filename);
+	lock_file = strdup(mkpath("%s.lock", config_filename));
+
 	/*
 	 * Since "key" actually contains the section name and the real
 	 * key name separated by a dot, we have to know where the dot is.
@@ -610,7 +619,7 @@ int git_config_set_multivar(const char* key, const char* value,
 		 * As a side effect, we make sure to transform only a valid
 		 * existing config file.
 		 */
-		if (git_config(store_aux)) {
+		if (git_config_from_file(store_aux, config_filename)) {
 			fprintf(stderr, "invalid config file\n");
 			free(store.key);
 			if (store.value_regex != NULL) {

t/Makefile

@@ -19,7 +19,7 @@ endif
 all: $(T) clean
 
 $(T):
-	@echo "*** $@ ***"; '$(SHELL_PATH_SQ)' $@ $(GIT_TEST_OPTS)
+	@echo "*** $@ ***"; GIT_CONFIG=.git/config '$(SHELL_PATH_SQ)' $@ $(GIT_TEST_OPTS)
 
 clean:
 	rm -fr trash

t/t1300-repo-config.sh

@@ -309,5 +309,29 @@ EOF
 
 test_expect_success 'new variable inserts into proper section' 'cmp .git/config expect'
 
+cat > other-config << EOF
+[ein]
+	bahn = strasse
+EOF
+
+cat > expect << EOF
+ein.bahn=strasse
+EOF
+
+GIT_CONFIG=other-config git-repo-config -l > output
+
+test_expect_success 'alternative GIT_CONFIG' 'cmp output expect'
+
+GIT_CONFIG=other-config git-repo-config anwohner.park ausweis
+
+cat > expect << EOF
+[ein]
+	bahn = strasse
+[anwohner]
+	park = ausweis
+EOF
+
+test_expect_success '--set in alternative GIT_CONFIG' 'cmp other-config expect'
+
 test_done