userdiff: skip textconv caching when not in a repository

The textconv caching system uses git-notes to store its cache entries.
But if you're using "diff --no-index" outside of a repository, then
obviously that isn't going to work.

Since caching is just an optimization, it's OK for us to skip it.
However, the current behavior is much worse: we call notes_cache_init()
which tries to look up the ref, and the low-level ref code hits a BUG(),
killing the program. Instead, we should notice before setting up the
cache that it there's no repository, and just silently skip it.

Reported-by: Paweł Dominiak <dominiak.pawel@gmail.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/userdiff.c b/userdiff.c
index e399543..fce3a31 100644
--- a/userdiff.c
+++ b/userdiff.c
@@ -3,6 +3,7 @@
 #include "userdiff.h"
 #include "attr.h"
 #include "strbuf.h"
+#include "environment.h"
 
 static struct userdiff_driver *drivers;
 static int ndrivers;
@@ -460,7 +461,8 @@ struct userdiff_driver *userdiff_get_textconv(struct repository *r,
 	if (!driver->textconv)
 		return NULL;
 
-	if (driver->textconv_want_cache && !driver->textconv_cache) {
+	if (driver->textconv_want_cache && !driver->textconv_cache &&
+	    have_git_dir()) {
 		struct notes_cache *c = xmalloc(sizeof(*c));
 		struct strbuf name = STRBUF_INIT;