git-gui: Display a progress bar during blame annotation gathering

git-gui: Display a progress bar during blame annotation gathering

Computing the blame records for a large file with a long project
history can take git a while to run; traditionally we have shown
a little meter in the status area of our blame viewer that lets
the user know how many lines have been finished, and how far we
are through the process.

Usually such progress indicators are drawn with a little progress
bar in the window, where the bar shows how much has been completed
and hides itself when the process is complete.  I'm using a very
simple hack to do that: draw a canvas with a filled rectangle.

Of course the time remaining has absolutely no relationship to the
progress meter.  It could take very little time for git-blame to get
the first 90% of the file, and then it could take many times that to
get the remaining 10%.  So the progress meter doesn't really have any
sort of assurances that it relates to the true progress of the work.
But in practice on some ugly history it does seem to hold a reasonable
indicator to the completion status.  Besides, its amusing to watch and
that keeps the user from realizing git is being somewhat slow.

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