git p4: avoid expanding client paths in chdir

git p4: avoid expanding client paths in chdir

The generic chdir() helper sets the PWD environment
variable, as that is what is used by p4 to know its
current working directory.  Normally the shell would
do this, but in git-p4, we must do it by hand.

However, when the path contains a symbolic link,
os.getcwd() will return the physical location.  If the
p4 client specification includes symlinks, setting PWD
to the physical location causes p4 to think it is not
inside the client workspace.  It complains, e.g.

    Path /vol/bar/projects/foo/... is not under client root /p/foo

One workaround is to use AltRoots in the p4 client specification,
but it is cleaner to handle it directly in git-p4.

Other uses of chdir still require setting PWD to an
absolute path so p4 features like P4CONFIG work.  See
bf1d68f (git-p4: use absolute directory for PWD env
var, 2011-12-09).

[ pw: tweak patch and commit message ]

Thanks-to: John Keeping <john@keeping.me.uk>
Signed-off-by: Pete Wyckoff <pw@padd.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>