userdiff: improve Fortran xfuncname regex

The third part of the Fortran xfuncname regex wants to match the
beginning of a subroutine or function, so it allows for all characters
except `'`, `"` or whitespace before the keyword 'function' or
'subroutine'. This is meant to match the 'recursive', 'elemental' or
'pure' keywords, as well as function return types, and to prevent
matches inside strings.

However, the negated set does not contain the `!` comment character,
so a line with an end-of-line comment containing the keyword 'function' or
'subroutine' followed by another word is mistakenly chosen as a hunk header.

Improve the regex by adding `!` to the negated set.

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>