gitk: Simplify the code for finding commits

gitk: Simplify the code for finding commits

This unifies findmore and findmorerev, and adds the ability to do
a search with or without wrap around from the end of the list of
commits to the beginning (or vice versa for reverse searches).
findnext and findprev are gone, and the buttons and keys for searching
all call dofind now.  dofind doesn't unmark the matches to start with.
Shift-up and shift-down are back by popular request, and the searches
they do don't wrap around.  The other keys that do searches (/, ?,
return, M-f) do wrapping searches except for M-g.

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