core: use os.getcwdu() as our core.getcwd() implementation

decode()/encode() is lossy when using non-utf-8 filesystems.
Use getcwdu() to avoid an extra conversion.

Related-to: #305
Signed-off-by: David Aguilar <davvid@gmail.com>
cola: use core.getcwd() everywhere

Replace all uses of os.getcwd() with core.getcwd() so that we get the
proper unicode-friendly behavior.

Related-to: #305
Signed-off-by: David Aguilar <davvid@gmail.com>