Merge branch 'kg/external-diff-save-env'

The code to drive GIT_EXTERNAL_DIFF command relied on the string
returned from getenv() to be non-volatile, which is not true, that
has been corrected.

* kg/external-diff-save-env:
  diff: ensure correct lifetime of external_diff_cmd

diff.c

@@ -493,7 +493,7 @@ 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;