vartree: skip env-update if no updates were merged

This speeds up virtual/ installs by not constantly re-running env-update.

BUG=None
TEST=CQ passes

Change-Id: I850dcc97f037232892a509a94fed3d5c3602c9c6
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/portage_tool/+/3558169
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Cindy Lin <xcl@google.com>
(cherry picked from commit 87ac3566ebb7155a57876d345849bd0fd6878c0e)
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/portage_tool/+/3614737
Commit-Queue: Cindy Lin <xcl@google.com>
Auto-Submit: Mike Frysinger <vapier@chromium.org>
1 file changed