t9902: disable test on the list of merge-strategies under GETTEXT_POISON

The code to learn the list of merge strategies from the output of
"git merge -s help" forces C locale, so that it can notice the
message shown to indicate where the list starts in the output.

However, GETTEXT_POISON build corrupts its output even when run in
the C locale, and we cannot expect this test to succeed.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

t/t9902-completion.sh

@@ -1511,7 +1511,7 @@ test_expect_success 'sourcing the completion script clears cached porcelain comm
 	verbose test -z "$__git_porcelain_commands"
 '
 
-test_expect_success 'sourcing the completion script clears cached merge strategies' '
+test_expect_success !GETTEXT_POISON 'sourcing the completion script clears cached merge strategies' '
 	__git_compute_merge_strategies &&
 	verbose test -n "$__git_merge_strategies" &&
 	. "$GIT_BUILD_DIR/contrib/completion/git-completion.bash" &&