apply, fmt-merge-msg: use relative filenames

apply, fmt-merge-msg: use relative filenames

Commit dbd0f5c7 (Files given on the command line are relative to $cwd,
2008-08-06) only fixed git-commit and git-tag. But, git-apply and
git-fmt-merge-msg didn't get the update and exhibit the same behavior.

Fix them and add tests for "apply --build-fake-ancestor" and
"fmt-merge-msg -F".

Signed-off-by: Stephen Boyd <redacted>
Signed-off-by: Junio C Hamano <redacted>