t7810: correct --no-index test

t7810: correct --no-index test

GIT_CEILING_DIRECTORIES doesn't prevent chdir up into another directory
while looking for a repository directory if it is equal to the current
directory.  Because of this, the test which claims to test the git grep
--no-index command outside of a repository actually tests it inside of a
repository.  The test_must_fail assertions still pass because the git
grep only looks at untracked files and therefore no file matches, but
not because it's run outside of a repository as it was originally
intended.

Set the GIT_CEILING_DIRECTORIES environment variable to the parent
directory of the directory in which the git grep command is executed, to
make sure it is actually run outside of a git repository.

In addition, the && chain was broken in a couple of places in the same
test, fix that.

Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>