config: factor out config file stack management

Because a config callback may start parsing a new file, the
global context regarding the current config file is stored
as a stack. Currently we only need to manage that stack from
git_config_from_file. Let's factor it out to allow new
sources of config data.
Signed-off-by: NHeiko Voigt <hvoigt@hvoigt.net>
Acked-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

config.c

@@ -896,6 +896,32 @@ int git_default_config(const char *var, const char *value, void *dummy)
 	return 0;
 }
 
+/*
+ * The fields f and name of top need to be initialized before calling
+ * this function.
+ */
+static int do_config_from(struct config_file *top, config_fn_t fn, void *data)
+{
+	int ret;
+
+	/* push config-file parsing state stack */
+	top->prev = cf;
+	top->linenr = 1;
+	top->eof = 0;
+	strbuf_init(&top->value, 1024);
+	strbuf_init(&top->var, 1024);
+	cf = top;
+
+	ret = git_parse_file(fn, data);
+
+	/* pop config-file parsing state stack */
+	strbuf_release(&top->value);
+	strbuf_release(&top->var);
+	cf = top->prev;
+
+	return ret;
+}
+
 int git_config_from_file(config_fn_t fn, const char *filename, void *data)
 {
 	int ret;
@@ -905,22 +931,10 @@ int git_config_from_file(config_fn_t fn, const char *filename, void *data)
 	if (f) {
 		config_file top;
 
-		/* push config-file parsing state stack */
-		top.prev = cf;
 		top.f = f;
 		top.name = filename;
-		top.linenr = 1;
-		top.eof = 0;
-		strbuf_init(&top.value, 1024);
-		strbuf_init(&top.var, 1024);
-		cf = ⊤
-
-		ret = git_parse_file(fn, data);
-
-		/* pop config-file parsing state stack */
-		strbuf_release(&top.value);
-		strbuf_release(&top.var);
-		cf = top.prev;
+
+		ret = do_config_from(&top, fn, data);
 
 		fclose(f);
 	}