chroot: set PATH when inside the chroot

chroot: set PATH when inside the chroot

The only possible value for PATH inside the chroot is "/bin" because
that's where all the binaries are installed.

Sure, /usr/bin/... works, but that's only because of the symlink
"/usr -> .".  Anything else left over in the PATH from outside the
chroot is just not valid.

Although non-existent directories in PATH do get ignored, and at
least one of "/bin" or "/usr/bin" ought to be in there already,
don't depend on that and just always set PATH to /bin when running
inside the chroot.

The only scripts that can run inside the chroot are git-shell-verify
and the pre-receive, update and post-receive hooks.  None of the
others can ever run inside the chroot -- at least not the chroot that
Girocco sets up for ssh pushing.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>