Move 'rev-parse --git-dir' test to t1500

Move 'rev-parse --git-dir' test to t1500

Commit 72183cb2 (Fix gitdir detection when in subdir of
gitdir, 2009-01-16) added a test to 't1501-worktree' to check the
behaviour of 'git rev-parse --git-dir' in a special case.  However,
t1501 is about testing separate work tree setups, and not about basic
'rev-parse' functionality, which is tested in t1500-rev-parse.
Therefore, this patch moves that test to t1500.

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>