completion: zsh: fix file completion regression

Turns out we always need to set the ignored prefix (compset) to have
similar behavior as in default Bash.

The issue can be seen with:

  git show master:<tab>

Commit 94b2901cfe wrongly removed it.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.zsh

@@ -116,6 +116,7 @@ __gitcomp_file ()
 {
 	emulate -L zsh
 
+	compset -P '*[=:]'
 	compadd -f -p "${2-}" -- ${(f)1} && _ret=0
 }