Progress reporting has gotten complicated, and has had a couple bugs.
Factor out progress-related behavior to a new progressTracker type, and
use this to implement some unit tests.
Also rename some methods to remove stuttering, and reorganize the code
to be more logical.
Fixesgolang/go#40527
Change-Id: I93d53a67982460e7171f892021e99f4523fe3e5d
Reviewed-on: https://go-review.googlesource.com/c/tools/+/247407
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>
