mergetool: dissect strings with shell variable magic instead of `expr`

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. Do so with `expr`.

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

git-mergetool.sh

@@ -228,9 +228,8 @@ stage_submodule () {
 }
 
 checkout_staged_file () {
-	tmpfile=$(expr \
-		"$(git checkout-index --temp --stage="$1" "$2" 2>/dev/null)" \
-		: '\([^	]*\)	')
+	tmpfile="$(git checkout-index --temp --stage="$1" "$2" 2>/dev/null)" &&
+	tmpfile=${tmpfile%%'	'*}
 
 	if test $? -eq 0 && test -n "$tmpfile"
 	then
@@ -255,13 +254,16 @@ merge_file () {
 		return 1
 	fi
 
-	if BASE=$(expr "$MERGED" : '\(.*\)\.[^/]*$')
-	then
-		ext=$(expr "$MERGED" : '.*\(\.[^/]*\)$')
-	else
+	# extract file extension from the last path component
+	case "${MERGED##*/}" in
+	*.*)
+		ext=.${MERGED##*.}
+		BASE=${MERGED%"$ext"}
+		;;
+	*)
 		BASE=$MERGED
 		ext=
-	fi
+	esac
 
 	mergetool_tmpdir_init
 
@@ -406,7 +408,7 @@ main () {
 		-t|--tool*)
 			case "$#,$1" in
 			*,*=*)
-				merge_tool=$(expr "z$1" : 'z-[^=]*=\(.*\)')
+				merge_tool=${1#*=}
 				;;
 			1,*)
 				usage ;;