get_pwd_cwd(): Do not trust st_dev/st_ino blindly

get_pwd_cwd(): Do not trust st_dev/st_ino blindly

10c4c88 (Allow add_path() to add non-existent directories to the path,
2008-07-21) introduced get_pwd_cwd() function in order to favor $PWD when
getenv("PWD") and getcwd() refer to the same directory but are different
strings (e.g. the former gives a nicer looking name via a symbolic link to
an uglier looking automounted path). The function tried to determine if
two directories are the same by running stat(2) on both and comparing
ino/dev fields.

Unfortunately, stat() does not fill any ino or dev fields in msysgit.  But
there is a telltale: both ino and dev are 0 when they are not filled
correctly, so let's be extra cautious.

This happens to fix a bug in "get-receive-pack working_directory/" when
the GIT_DIR would not be set correctly due to absolute_path(".")
returning the wrong value.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Acked-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>