gitk: add horizontal scrollbar to the commit list pane

gitk: add horizontal scrollbar to the commit list pane

When many branches and tags decorate the same commit, the ref labels
push the commit description far to the right, often out of the visible
area of the left pane.  The canvas widget already tracked the maximum
x extent via canvxmax and updated its scrollregion accordingly, but
there was no scrollbar wired up to let the user reach that content.

Add a horizontal scrollbar (.tf.histframe.cxsb) below the three-pane
history area, connected to the left canvas (canv) via its xscrollcommand
and xview.

No behaviour is changed for the author (canv2) or date (canv3) panes;
they continue to scroll only vertically in lock-step as before.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Rob McDonald <rob.a.mcdonald@gmail.com>