fetch2/git: Work around git confusion between foo.git and foo repositories

fetch2/git: Work around git confusion between foo.git and foo repositories

If you have foo and foo.git in GITDIR, the two can end up being confused
by git with some horrible union of the two being cloned. This adds
a workaround to avoid this happening until git 1.7.9.2 onwards is
common enough for this to be removed. We use a symlink to hide
the directories we don't want git to know about.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>