gitk: show unescaped file names on 'rename' and 'copy' lines

gitk: show unescaped file names on 'rename' and 'copy' lines

When a file is selected in the file list, the diff window scrolls to the
corresponding section. The administrative data needed for this purpose
is extracted from the 'rename from', 'rename to', and 'copy to' lines.
Escaped file names are unescaped for this purpose. However, the lines
shown in the diff window are left in the escaped form. This is not very
pleasing. Replace the escaped form by the unescaped form.

Add a section to treat the 'copy from' case.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>