git am --skip: clean the index while preserving local changes

In 3-way merge, "am" will let the index with unmerged path waiting
for us to resolve conflicts and continue. But if we want to --skip
instead, "am" refuses to continue because of the dirty index.

With this patch, "am" will clean the index without touching files
locally modified, before continuing.

Signed-off-by: Olivier Marin <dkr@freesurf.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-am.sh

@@ -202,8 +202,15 @@ then
 	die "previous rebase directory $dotest still exists but mbox given."
 	resume=yes
 
-	case "$abort" in
-	t)
+	case "$skip,$abort" in
+	t,)
+		git rerere clear
+		git read-tree --reset -u HEAD HEAD
+		orig_head=$(cat "$GIT_DIR/ORIG_HEAD")
+		git reset HEAD
+		git update-ref ORIG_HEAD $orig_head
+		;;
+	,t)
 		git rerere clear
 		git read-tree --reset -u HEAD ORIG_HEAD
 		git reset ORIG_HEAD
@@ -297,7 +304,6 @@ last=`cat "$dotest/last"`
 this=`cat "$dotest/next"`
 if test "$skip" = t
 then
-	git rerere clear
 	this=`expr "$this" + 1`
 	resume=
 fi

t/t4151-am-abort.sh

@@ -14,7 +14,7 @@ test_expect_success setup '
 	git add file-1 file-2 &&
 	git commit -m initial &&
 	git tag initial &&
-	for i in 2 3 4 5
+	for i in 2 3 4 5 6
 	do
 		echo $i >>file-1 &&
 		test_tick &&
@@ -32,7 +32,7 @@ do
 		git reset --hard initial &&
 		cp file-2-expect file-2 &&
 
-		test_must_fail git am$with3 000[124]-*.patch &&
+		test_must_fail git am$with3 000[1245]-*.patch &&
 		git log --pretty=tformat:%s >actual &&
 		for i in 3 2 initial
 		do
@@ -41,6 +41,13 @@ do
 		test_cmp expect actual
 	'
 
+	test_expect_success "am$with3 --skip continue after failed am$with3" '
+		test_must_fail git-am$with3 --skip >output &&
+		test "$(grep "^Applying" output)" = "Applying 6" &&
+		test_cmp file-2-expect file-2 &&
+		test ! -f .git/rr-cache/MERGE_RR
+	'
+
 	test_expect_success "am --abort goes back after failed am$with3" '
 		git-am --abort &&
 		git rev-parse HEAD >actual &&