Since git reset now gets chatty while resetting, we were getting errors
reported when a reset was done using the "reset branch to here" menu
item.  With this we now read the progress messages from git reset and
update a progress bar.  Because git reset outputs the progress messages
to standard error, and Tcl treats messages to standard error as error
messages, we have to invoke git reset via a shell and redirect standard
error into standard output.

This also fixes a bug in computing descendent heads when head ids
are changed via a reset.

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