git-completion.tcsh: fix redirect with noclobber

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>

contrib/completion/git-completion.tcsh

@@ -41,7 +41,7 @@ if ( ! -e ${__git_tcsh_completion_original_script} ) then
 	exit
 endif
 
-cat << EOF > ${__git_tcsh_completion_script}
+cat << EOF >! ${__git_tcsh_completion_script}
 #!bash
 #
 # This script is GENERATED and will be overwritten automatically.