For a keyboard addict like me some keys are still missing from
gitk. Especially a key to select a commit when no commit is selected,
like just after startup. While we're at it, complete the bindings for
moving the view seperately from the selected line. Currently, the up
and down keys act on the selected line while pageup and pagedown act
on the commits viewed.

The idea is to have to normal keys change the selected line:
  - Home selects first commit
  - End selects last commit
  - Up selects previous commit
  - Down selects next commit
  - PageUp moves selected line one page up
  - PageDown moves selected line one page down
...and together with the Control key, it moves the commits view:
  - Control-Home views first page of commits
  - Control-End views last page of commits
  - Control-Up moves commit view one line up
  - Control-Down moves commit view one line down
  - Control-PageUp moves commit view one page up
  - Control-PageDown moves commit view one page down
Signed-off-By: NRutger Nijlunsing <gitk@tux.tmfweb.nl>

and with some cleanups and simplifications...
Signed-off-by: NPaul Mackerras <paulus@samba.org>