[roll] Roll fuchsia [update] implement `update revert`

This change implements `update revert`, which can be used to revert the
update and fall back to the previous slot.

Test: fx test -o update-lib-tests
Test: fx test -o update-integration-tests
Test (manual): Observed we booted into zedboot after running `update
revert`.
Test (manual): Observed we booted into the alternate slot when adding a
timer to make commits hang, and reverting before the commit.
Original-Fixed: 64590

Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/461522
Original-Revision: 82da73281da9c007b07ecaad0cf3cd22e4104cc6
GitOrigin-RevId: 047a902bb7c7af3c932aaae70f7349e92579ad06
Change-Id: I86d58a0c79d233b4a064246b1345452bcc6df678
diff --git a/stem b/stem
index 983b02b..a199adc 100644
--- a/stem
+++ b/stem
@@ -10,7 +10,7 @@
              githooks=""
              remote="https://fuchsia.googlesource.com/fuchsia"
              gerrithost="https://fuchsia-review.googlesource.com"
-             revision="b66a9436dd3b159d466bbb6d65c0c4fb044f2b2c"/>
+             revision="82da73281da9c007b07ecaad0cf3cd22e4104cc6"/>
   </projects>
   <hooks>
     <hook name="install-fx"