git-add: no need for -f when resolving a conflict in already tracked path

When a path F that matches ignore pattern has a conflict, "git add F"
insisted the -f option be given, which did not make sense.  It would have
required -f when the path was originally added, but when resolving a
conflict, it already is tracked.

So this should work (and does):

  $ echo file >.gitignore
  $ echo content >file
  $ git add -f file ;# need -f because we are adding new path
  $ echo more content >>file
  $ git add file ;# don't need -f; it is not actually an "other" file

This is handled under the hood by the COLLECT_IGNORED option to
read_directory. When that code finds an ignored file, it checks the
index to make sure it is not actually a tracked file. However, the test
it uses does not take into account unmerged entries, and considers them
to still be ignored. "git ls-files" uses a more elaborate test and gets
the right answer and the same test should be used here.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

dir.c

@@ -396,7 +396,7 @@ static struct dir_entry *dir_add_name(struct dir_struct *dir, const char *pathna
 
 static struct dir_entry *dir_add_ignored(struct dir_struct *dir, const char *pathname, int len)
 {
-	if (cache_name_pos(pathname, len) >= 0)
+	if (!cache_name_is_other(pathname, len))
 		return NULL;
 
 	ALLOC_GROW(dir->ignored, dir->ignored_nr+1, dir->ignored_alloc);

t/t3700-add.sh

@@ -230,4 +230,16 @@ test_expect_success BSLASHPSPEC "git add 'fo\\[ou\\]bar' ignores foobar" '
 	! ( git ls-files foobar | grep foobar )
 '
 
+test_expect_success 'git add to resolve conflicts on otherwise ignored path' '
+	git reset --hard &&
+	H=$(git rev-parse :1/2/a) &&
+	(
+		echo "100644 $H 1	track-this"
+		echo "100644 $H 3	track-this"
+	) | git update-index --index-info &&
+	echo track-this >>.gitignore &&
+	echo resolved >track-this &&
+	git add track-this
+'
+
 test_done