unpack-trees: fix grammar for untracked files in directories

Noticed-by: David Turner <dturner@twosigma.com>
Signed-off-by: Stefan Beller <sbeller@google.com>
Reviewed-by: David Turner <dturner@twosigma.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7609-merge-co-error-msgs.sh

@@ -105,7 +105,7 @@ test_expect_success 'not uptodate file porcelain checkout error' '
 '
 
 cat >expect <<\EOF
-error: Updating the following directories would lose untracked files in it:
+error: Updating the following directories would lose untracked files in them:
 	rep
 	rep2
 

unpack-trees.c

@@ -78,7 +78,7 @@ void setup_unpack_trees_porcelain(struct unpack_trees_options *opts,
 		xstrfmt(msg, cmd, cmd);
 
 	msgs[ERROR_NOT_UPTODATE_DIR] =
-		_("Updating the following directories would lose untracked files in it:\n%s");
+		_("Updating the following directories would lose untracked files in them:\n%s");
 
 	if (!strcmp(cmd, "checkout"))
 		msg = advice_commit_before_merge