check-non-portable-shell: detect obsolescent egrep/fgrep

check-non-portable-shell: detect obsolescent egrep/fgrep

GNU grep deprecated `egrep` and `fgrep` with release 2.5.3 in 2007.
As of release 3.8 in 2022, those commands warn[1] that they are
obsolescent. Now that all the Git test scripts have been scrubbed of
uses of `egrep` and `fgrep`, make `check-non-portable-shell` complain
about them to prevent new instances from creeping back into the project.

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

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>