mergetool: use shell variable magic instead of `awk`

git-mergetool spawns an enormous amount of processes. For this reason,
the test script, t7610, is exceptionally slow, in particular, on
Windows. Most of the processes are invocations of git. There are
also some that can be replaced with shell builtins. Avoid repeated
calls of `git ls-files` and `awk`.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-mergetool.sh

@@ -279,15 +279,30 @@ merge_file () {
 	REMOTE="$MERGETOOL_TMPDIR/${BASE}_REMOTE_$$$ext"
 	BASE="$MERGETOOL_TMPDIR/${BASE}_BASE_$$$ext"
 
-	base_mode=$(git ls-files -u -- "$MERGED" | awk '{if ($3==1) print $1;}')
-	local_mode=$(git ls-files -u -- "$MERGED" | awk '{if ($3==2) print $1;}')
-	remote_mode=$(git ls-files -u -- "$MERGED" | awk '{if ($3==3) print $1;}')
+	base_mode= local_mode= remote_mode=
+
+	# here, $IFS is just a LF
+	for line in $f
+	do
+		mode=${line%% *}		# 1st word
+		sha1=${line#"$mode "}
+		sha1=${sha1%% *}		# 2nd word
+		case "${line#$mode $sha1 }" in	# remainder
+		'1	'*)
+			base_mode=$mode
+			;;
+		'2	'*)
+			local_mode=$mode local_sha1=$sha1
+			;;
+		'3	'*)
+			remote_mode=$mode remote_sha1=$sha1
+			;;
+		esac
+	done
 
 	if is_submodule "$local_mode" || is_submodule "$remote_mode"
 	then
 		echo "Submodule merge conflict for '$MERGED':"
-		local_sha1=$(git ls-files -u -- "$MERGED" | awk '{if ($3==2) print $2;}')
-		remote_sha1=$(git ls-files -u -- "$MERGED" | awk '{if ($3==3) print $2;}')
 		describe_file "$local_mode" "local" "$local_sha1"
 		describe_file "$remote_mode" "remote" "$remote_sha1"
 		resolve_submodule_merge