gitk: Synchronize highlighting in file view for 'f' and 'b' commands

gitk: Synchronize highlighting in file view for 'f' and 'b' commands

This is based on a patch by Eric Raible <raible@gmail.com>, but does
things a bit more simply.

Previously, 'b', backspace, and delete all did the same thing.
This changes 'b' to perform the inverse of 'f'.  And both of
them now highlight the filename of the currently diff.

This makes it easier to review and navigate the diffs associated
with a particular commit using only f, b, and space because the
filename of the currently display diff will be dynamically
highlighted.

Signed-off-by: Paul Mackerras <paulus@samba.org>