untracked cache: don't open non-existent .gitignore
untracked cache: don't open non-existent .gitignore

This cuts down a signficant number of open(.gitignore) because most
directories usually don't have .gitignore files.

Signed-off-by: Nguyễn Thái Ngọc Duy <024f400cd820cb2ddbe54c7fecfd01e45f30bb96@gmail.com>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git