git-gui: Use progress bar while resetting/aborting files

git-gui: Use progress bar while resetting/aborting files

Resetting a large number of files on a slow filesystem can take
considerable time, just as switching branches in such a case can
take more than two seconds.  We now take advantage of the progress
meter output by read-tree and show it in the main window status
bar, just like we do during checkout (branch switch).

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>