tests: fix negated test_i18ngrep calls

The helper function test_i18ngrep pretends that it found the expected
results when it is running under GETTEXT_POISON. For this reason, it must
not be used negated like so

   ! test_i18ngrep foo bar

because the test case would fail under GETTEXT_POISON. The function offers
a special syntax to test that a pattern is *not* found:

   test_i18ngrep ! foo bar

Convert incorrect uses to this syntax.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Git 2.1

Signed-off-by: Junio C Hamano <gitster@pobox.com>
tree-walk: be more specific about corrupt tree errors

When the tree-walker runs into an error, it just calls
die(), and the message is always "corrupt tree file".
However, we are actually covering several cases here; let's
give the user a hint about what happened.

Let's also avoid using the word "corrupt", which makes it
seem like the data bit-rotted on disk. Our sha1 check would
already have found that. These errors are ones of data that
is malformed in the first place.