Add git-describe test for "verify annotated tag names on output"

Back in 212945d4 ("Teach git-describe to verify annotated tag names
before output") I taught git-describe to output the name shown in the
"tag" header of an annotated tag, rather than the name it is actually
stored under in this repository's ref namespace.

This test case verifies this is working correctly by renaming the ref
for an annotated tag to a different name that what is recorded in the
tag body, and verifying that tag is returned.  We also verify there is
a message shown on stderr to inform the user that the tag is possibly
stored under the wrong name locally.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t6120-describe.sh

@@ -15,8 +15,9 @@ test_description='test describe
 check_describe () {
 	expect="$1"
 	shift
-	R=$(git describe "$@")
+	R=$(git describe "$@" 2>err.actual)
 	S=$?
+	cat err.actual >&3
 	test_expect_success "describe $*" '
 	test $S = 0 &&
 	case "$R" in
@@ -98,6 +99,20 @@ check_describe B --tags HEAD^^2^
 
 check_describe B-0-* --long HEAD^^2^
 
+test_expect_success 'rename tag A to Q locally' '
+	mv .git/refs/tags/A .git/refs/tags/Q
+'
+cat - >err.expect <<EOF
+warning: tag 'A' is really 'Q' here
+EOF
+check_describe A-* HEAD
+test_expect_success 'warning was displayed for Q' '
+	git diff err.expect err.actual
+'
+test_expect_success 'rename tag Q back to A' '
+	mv .git/refs/tags/Q .git/refs/tags/A
+'
+
 test_expect_success 'pack tag refs' 'git pack-refs'
 check_describe A-* HEAD