mv test: recreate mod/ directory instead of relying on stale copy

mv test: recreate mod/ directory instead of relying on stale copy

The tests for 'git mv moves a submodule' functionality often run
commands like

	git mv sub mod/sub

to move a submodule into a subdirectory.  Just like plain /bin/mv,
this is supposed to succeed if the mod/ parent directory exists
and fail if it doesn't exist.

Usually these tests mkdir the parent directory beforehand, but some
instead rely on it being left behind by previous tests.

More precisely, when 'git reset --hard' tries to move to a state where
mod/sub is not present any more, it would perform the following
operations:

	rmdir("mod/sub")
	rmdir("mod")

The first fails with ENOENT because the test script removed mod/sub
with "rm -rf" already, so 'reset --hard' doesn't bother to move on to
the second, and the mod/ directory is kept around.

Better to explicitly remove and re-create the mod/ directory so later
tests don't have to depend on the directory left behind by the earlier
ones at all (making it easier to rearrange or skip some tests in the
file or to tweak 'reset --hard' behavior without breaking unrelated
tests).

Noticed while testing a patch that fixes the reset --hard behavior
described above.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Reviewed-by: Ronnie Sahlberg <sahlberg@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>