tests: try git apply from subdir of toplevel

tests: try git apply from subdir of toplevel

Make sure git apply can apply patches with paths relative to the
toplevel of a work tree, a subdirectory, or within the repository
metadata directory.

Relative paths are broken for most commands when run from a
subdirectory of $GIT_DIR, "git apply" being no exception.  The other
tests are meant to keep the demonstration of that company.

Based on a test by Duy.

Cc: Nguyễn Thái Ngọc Duy <redacted>
Signed-off-by: Jonathan Nieder <redacted>
Signed-off-by: Junio C Hamano <redacted>