tcsh users who happen to have 'set noclobber' elsewhere in their
~/.tcshrc or ~/.cshrc startup files get a 'File exist' error, and
the tcsh completion file doesn't get generated/updated.
Adding a `!` in the redirect works correctly for both clobber (default)
and 'set noclobber' users.
Reviewed-by: Christian Couder <christian.couder@gmail.com> Signed-off-by: Ariel Faigon <github.2009@yendor.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>