git-gui: Show a progress meter for checking out files

git-gui: Show a progress meter for checking out files

Sometimes switching between branches can take more than a second or
two, in which case `git checkout` would normally have shown a small
progress meter to the user on the terminal to let them know that we
are in fact working, and give them a reasonable idea of when we may
finish.

We now do obtain that progress meter from read-tree -v and include
it in our main window's status bar.  This allows users to see how
many files we have checked out, how many remain, and what percentage
of the operation is completed.  It should help to keep users from
getting bored during a large checkout operation.

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