checkout,merge: loosen overwriting untracked file check based on info/exclude

checkout,merge: loosen overwriting untracked file check based on info/exclude

Back in 1127148 (Loosen "working file will be lost" check in
Porcelain-ish - 2006-12-04), git-checkout.sh learned to quietly
overwrite ignored files. Howver the code only took .gitignore files
into account.

Standard ignored files include all specified in .gitignore files in
working directory _and_ $GIT_DIR/info/exclude. This patch makes sure
ignored files in info/exclude can also be overwritten automatically in
the spirit of the original patch.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>