t2400: rewrite regex to avoid unintentional PCRE

t2400: rewrite regex to avoid unintentional PCRE

Replace all cases of `\s` with ` ` as it is not part of POSIX BRE or ERE
and therefore not all versions of grep handle it.

For the same reason all cases of `\S` are replaced with `[^ ]`. It is
not an exact replacement but it is close enough for this use case.

Also, do not write `\+` in BRE and expect it to mean 1 or more;
it is a GNU extension that may not work everywhere.

Remove `.*` from the end of a pattern that is not right-anchored.

Signed-off-by: Jacob Abel <redacted>
Helped-by: Junio C Hamano <redacted>
Signed-off-by: Junio C Hamano <redacted>