completion: use __gitcompadd for __gitcomp_file

Like the rest of the script does; let's not access COMPREPLY directly.

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

Author: Felipe Contreras

contrib/completion/git-completion.bash

@@ -252,7 +252,7 @@ __gitcomp_file ()
 	# since tilde expansion is not applied.
 	# This means that COMPREPLY will be empty and Bash default
 	# completion will be used.
-	COMPREPLY=($(compgen -P "${2-}" -W "$1" -- "${3-$cur}"))
+	__gitcompadd "$1" "${2-}" "${3-$cur}" ""
 
 	# Tell Bash that compspec generates filenames.
 	compopt -o filenames 2>/dev/null