findmissing: Convert active git operations to class

Using a class for active git operations enables us to store repository
and branch states as part of a class instance. We can use that information
to reduce the number of git operations and thus improve performance
significantly.

BUG=b:177099511
TEST=Run robot

Change-Id: I927ad1c02cda685a31fdebc7ed1b5f136902df4a
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/2637918
Reviewed-by: Curtis Malainey <cujomalainey@chromium.org>
Commit-Queue: Guenter Roeck <groeck@chromium.org>
Tested-by: Guenter Roeck <groeck@chromium.org>
5 files changed