completion: add new __gitcompadd helper

The idea is to never touch the COMPREPLY variable directly.

This allows other completion systems (i.e. zsh) to override
__gitcompadd, and do something different instead.

Also, this allows further optimizations down the line.

There should be no functional changes.

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

contrib/completion/git-completion.bash

@@ -195,6 +195,11 @@ _get_comp_words_by_ref ()
 }
 fi
 
+__gitcompadd ()
+{
+	COMPREPLY=($(compgen -W "$1" -P "$2" -S "$4" -- "$3"))
+}
+
 # Generates completion reply with compgen, appending a space to possible
 # completion words, if necessary.
 # It accepts 1 to 4 arguments:
@@ -211,9 +216,7 @@ __gitcomp ()
 		;;
 	*)
 		local IFS=$'\n'
-		COMPREPLY=($(compgen -P "${2-}" \
-			-W "$(__gitcomp_1 "${1-}" "${4-}")" \
-			-- "$cur_"))
+		__gitcompadd "$(__gitcomp_1 "${1-}" "${4-}")" "${2-}" "$cur_" ""
 		;;
 	esac
 }
@@ -230,7 +233,7 @@ __gitcomp ()
 __gitcomp_nl ()
 {
 	local IFS=$'\n'
-	COMPREPLY=($(compgen -P "${2-}" -S "${4- }" -W "$1" -- "${3-$cur}"))
+	__gitcompadd "$1" "${2-}" "${3-$cur}" "${4- }"
 }
 
 # Generates completion reply with compgen from newline-separated possible
@@ -1825,7 +1828,7 @@ _git_config ()
 		local remote="${prev#remote.}"
 		remote="${remote%.fetch}"
 		if [ -z "$cur" ]; then
-			COMPREPLY=("refs/heads/")
+			__gitcompadd "refs/heads/" "" "" ""
 			return
 		fi
 		__gitcomp_nl "$(__git_refs_remotes "$remote")"