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>

t/t4042-diff-textconv-caching.sh

@@ -118,4 +118,26 @@ test_expect_success 'log notes cache and still use cache for -p' '
 	git log --no-walk -p refs/notes/textconv/magic HEAD
 '
 
+test_expect_success 'caching is silently ignored outside repo' '
+	mkdir -p non-repo &&
+	echo one >non-repo/one &&
+	echo two >non-repo/two &&
+	echo "* diff=test" >attr &&
+	test_expect_code 1 \
+	nongit git -c core.attributesFile="$PWD/attr" \
+		   -c diff.test.textconv="tr a-z A-Z <" \
+		   -c diff.test.cachetextconv=true \
+		   diff --no-index one two >actual &&
+	cat >expect <<-\EOF &&
+	diff --git a/one b/two
+	index 5626abf..f719efd 100644
+	--- a/one
+	+++ b/two
+	@@ -1 +1 @@
+	-ONE
+	+TWO
+	EOF
+	test_cmp expect actual
+'
+
 test_done

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;