gitweb: allow space as delimiter in mime.types

in openSUSE /etc/mime.types has only spaces. I don't know if there's
a canonical reference that says that only tabs are allowed. Mutt at
least also accepts spaces. So make gitweb more liberal too.

Signed-off-by: Ludwig Nussel <redacted>
Acked-by: Jakub Narebski <redacted>
Acked-by: John 'Warthog9' Hawley <redacted>
Signed-off-by: Junio C Hamano <redacted>