Merge pull request git-for-windows#2026 from gitster/kg/external-diff…

…-save-env

diff: ensure correct lifetime of external_diff_cmd

diff.c

@@ -489,7 +489,7 @@ static const char *external_diff(void)
 
 	if (done_preparing)
 		return external_diff_cmd;
-	external_diff_cmd = getenv("GIT_EXTERNAL_DIFF");
+	external_diff_cmd = xstrdup_or_null(getenv("GIT_EXTERNAL_DIFF"));
 	if (!external_diff_cmd)
 		external_diff_cmd = external_diff_cmd_cfg;
 	done_preparing = 1;