do not overwrite files marked "assume unchanged"

A merge will fail gracefully if it needs to update files marked
"assume unchanged", but other similar commands will not. In
particular, checkout and rebase will silently overwrite changes to
such files.

This is a regression introduced in commit 1dcafcc (verify_uptodate():
add ce_uptodate(ce) test), which avoids lstat's during a merge, if the
index entry is up-to-date. If the CE_VALID flag is set, however, we
cannot trust CE_UPTODATE.

Signed-off-by: Clemens Buchacher <drizzd@aon.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t2106-update-index-assume-unchanged.sh

@@ -0,0 +1,24 @@
+#!/bin/sh
+
+test_description='git update-index --assume-unchanged test.
+'
+
+. ./test-lib.sh
+
+test_expect_success 'setup' \
+	': >file &&
+	 git add file &&
+	 git commit -m initial &&
+	 git branch other &&
+	 echo upstream >file &&
+	 git add file &&
+	 git commit -m upstream'
+
+test_expect_success 'do not switch branches with dirty file' \
+	'git reset --hard &&
+	 git checkout other &&
+	 echo dirt >file &&
+	 git update-index --assume-unchanged file &&
+	 test_must_fail git checkout master'
+
+test_done

unpack-trees.c

@@ -862,7 +862,7 @@ static int verify_uptodate_1(struct cache_entry *ce,
 {
 	struct stat st;
 
-	if (o->index_only || (!ce_skip_worktree(ce) && (o->reset || ce_uptodate(ce))))
+	if (o->index_only || (!((ce->ce_flags & CE_VALID) || ce_skip_worktree(ce)) && (o->reset || ce_uptodate(ce))))
 		return 0;
 
 	if (!lstat(ce->name, &st)) {