gitk: Select head of current branch by default

gitk: Select head of current branch by default

Instead of selecting the first commit that appears, this makes gitk
select the currently checked out head, if the user hasn't explicitly
selected some other commit by the time it appears.  If the head hasn't
appeared by the time the graph is complete, then we select the first
real commit.

This applies both for graph updates and when the graph is being read
in initially.

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