bash completion: Improve responsiveness of git-log completion

Junio noticed the bash completion has been taking a long time lately.
Petr Baudis tracked it down to 72e5e989b ("bash: Add space after
unique command name is completed.").  Tracing the code showed
we spent significant time inside of this loop within __gitcomp,
due to the string copying overhead.

  [28.146109654] _git common over
  [28.164791148] gitrefs in
  [28.280302268] gitrefs dir out
  [28.300939737] gitcomp in
  [28.308378112] gitcomp pre-case
* [28.313407453] gitcomp iter in
* [28.701270296] gitcomp iter out
  [28.713370786] out normal

Since __git_refs avoids this string copying by forking and using
echo we use the same trick here when we need to finish generating
the names for the caller.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -114,9 +114,20 @@ __git_ps1 ()
 	fi
 }
 
+__gitcomp_1 ()
+{
+	local c IFS=' '$'\t'$'\n'
+	for c in $1; do
+		case "$c$2" in
+		--*=*) printf %s$'\n' "$c$2" ;;
+		*.)    printf %s$'\n' "$c$2" ;;
+		*)     printf %s$'\n' "$c$2 " ;;
+		esac
+	done
+}
+
 __gitcomp ()
 {
-	local all c s=$'\n' IFS=' '$'\t'$'\n'
 	local cur="${COMP_WORDS[COMP_CWORD]}"
 	if [ $# -gt 2 ]; then
 		cur="$3"
@@ -124,21 +135,14 @@ __gitcomp ()
 	case "$cur" in
 	--*=)
 		COMPREPLY=()
-		return
 		;;
 	*)
-		for c in $1; do
-			case "$c$4" in
-			--*=*) all="$all$c$4$s" ;;
-			*.)    all="$all$c$4$s" ;;
-			*)     all="$all$c$4 $s" ;;
-			esac
-		done
+		local IFS=$'\n'
+		COMPREPLY=($(compgen -P "$2" \
+			-W "$(__gitcomp_1 "$1" "$4")" \
+			-- "$cur"))
 		;;
 	esac
-	IFS=$s
-	COMPREPLY=($(compgen -P "$2" -W "$all" -- "$cur"))
-	return
 }
 
 __git_heads ()