alias.c: replace `git_config()` with `git_config_get_string()`

Use `git_config_get_string()` instead of `git_config()` to take advantage of
the config-set API which provides a cleaner control flow.

Signed-off-by: Tanay Abhra <tanayabh@gmail.com>
Reviewed-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

alias.c

@@ -1,26 +1,13 @@
 #include "cache.h"
 
-static const char *alias_key;
-static char *alias_val;
-
-static int alias_lookup_cb(const char *k, const char *v, void *cb)
-{
-	const char *name;
-	if (skip_prefix(k, "alias.", &name) && !strcmp(name, alias_key)) {
-		if (!v)
-			return config_error_nonbool(k);
-		alias_val = xstrdup(v);
-		return 0;
-	}
-	return 0;
-}
-
 char *alias_lookup(const char *alias)
 {
-	alias_key = alias;
-	alias_val = NULL;
-	git_config(alias_lookup_cb, NULL);
-	return alias_val;
+	char *v = NULL;
+	struct strbuf key = STRBUF_INIT;
+	strbuf_addf(&key, "alias.%s", alias);
+	git_config_get_string(key.buf, &v);
+	strbuf_release(&key);
+	return v;
 }
 
 #define SPLIT_CMDLINE_BAD_ENDING 1