gitk: Unicode file name support

gitk: Unicode file name support

Assumes file names in git tree objects are UTF-8 encoded.

On most unix systems, the system encoding (and thus the TCL system
encoding) will be UTF-8, so file names will be displayed correctly.

On Windows, it is impossible to set the system encoding to UTF-8.
Changing the TCL system encoding (via 'encoding system ...', e.g. in the
startup code) is explicitly discouraged by the TCL docs.

Change gitk functions dealing with file names to always convert
from and to UTF-8.

Signed-off-by: Karsten Blees <blees@dcon.de>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>