fsmonitor: read from getcwd(), not the PWD environment variable

fsmonitor: read from getcwd(), not the PWD environment variable

Though the process has chdir'd to the root of the working tree, the
PWD environment variable is only guaranteed to be updated accordingly
if a shell is involved -- which is not guaranteed to be the case.
That is, if `/usr/bin/perl` is a binary, $ENV{PWD} is unchanged from
whatever spawned `git` -- if `/usr/bin/perl` is a trivial shell
wrapper to the real `perl`, `$ENV{PWD}` will have been updated to the
root of the working copy.

Update to read from the Cwd module using the `getcwd` syscall, not the
PWD environment variable.  The Cygwin case is left unchanged, as it
necessarily _does_ go through a shell.

Signed-off-by: Alex Vandiver <alexmv@dropbox.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>