t: convert fgrep usage to "grep -F"

t: convert fgrep usage to "grep -F"

Despite POSIX states that:

> The old egrep and fgrep commands are likely to be supported for many
> years to come as implementation extensions, allowing historical
> applications to operate unmodified.

GNU grep 3.8 started to warn[1]:

> The egrep and fgrep commands, which have been deprecated since
> release 2.5.3 (2007), now warn that they are obsolescent and should
> be replaced by grep -E and grep -F.

Prepare for their removal in the future.

[1]: https://lists.gnu.org/archive/html/info-gnu/2022-09/msg00001.html

Signed-off-by: Đoàn Trần Công Danh <congdanhqx@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>