gitk: Add a "reset branch to here" row context-menu operation

gitk: Add a "reset branch to here" row context-menu operation

This adds an entry to the menu that comes up when the user does a
right-click on a row.  The new entry allows the user to reset the
currently checked-out head to the commit for the row that they did
the right-click on.  The user has to select what type of reset to
do, and confirm the reset, via a dialog box that pops up.

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