test suite fails if sh != bash || tar != GNU tar

I've built git on several BSD (FreeBSD and DragonFlyBSD) systems, and
while it builds fine on both platforms, the test suite *fails* on
both.

I tracked most of the problems down to dependencies on bash and GNU
tar. I'm not sure yet what the specific differences are between BSD's
/bin/sh and bash (the two are separate in the BSD world), but it's
obvious what the problem is with tar: the git-archive test parses
datestamps printed out by tar, and the two tar's print datestamps
differently.

If I  set SHELL_PATH=/usr/local/bin/bash and TAR=gtar (the name of GNU
tar on BSD systems) I am able to get thru all the tests on FreeBSD
(this is with a checkout of the v1.5.2.3 tag).

On DragonFly there is another problem that I have not yet tracked
down. git-mailinfo fails on 0004 - it fails to extract *anything* from
the message.

Anyone running Linux or Mac OSX won't see these problems, because on
both platforms /bin/sh == bash and tar == GNU tar.

Cheers,

- David
-- 
If I have not seen farther, it is because I have stood in the
footsteps of giants.