git-am: Keep index in case of abort with dirty index

git am --abort resets the index unconditionally. But in case a previous
git am exited due to a dirty index it is preferable to keep that index.
Make it so.

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-am.sh

@@ -230,11 +230,14 @@ then
 		;;
 	,t)
 		git rerere clear
-		git read-tree --reset -u HEAD ORIG_HEAD
-		git reset ORIG_HEAD
+		test -f "$dotest/dirtyindex" || {
+			git read-tree --reset -u HEAD ORIG_HEAD
+			git reset ORIG_HEAD
+		}
 		rm -fr "$dotest"
 		exit ;;
 	esac
+	rm -f "$dotest/dirtyindex"
 else
 	# Make sure we are not given --skip, --resolved, nor --abort
 	test "$skip$resolved$abort" = "" ||
@@ -287,7 +290,11 @@ fi
 case "$resolved" in
 '')
 	files=$(git diff-index --cached --name-only HEAD --) || exit
-	test "$files" && die "Dirty index: cannot apply patches (dirty: $files)"
+	if test "$files"
+	then
+		: >"$dotest/dirtyindex"
+		die "Dirty index: cannot apply patches (dirty: $files)"
+	fi
 esac
 
 if test "$(cat "$dotest/utf8")" = t