Make the "mark branch dormant" instructions simpler to copy+paste.

I regularly run into this error for local branches corresponding to
merged CLs. Before, I had to manually type `git rebase --abort`
before copy-pasting the `git config branch.[...].dormant true` line
into my terminal. With this change, I can just copy-paste once to
execute both steps, then press up+up+enter to retry.

As a side benefit, Git newcomers won't have to look up how to abort
a rebase.

Anyone preferring the previous workflow can still elect to copy
just the `git config` line.

Bug: none
Change-Id: I516d4ed12c5111936cc65460ce119a43e5f86a8b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3570882
Auto-Submit: Chloe Pelling <cpelling@google.com>
Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>

git_rebase_update.py

@@ -205,6 +205,7 @@ def rebase_branch(branch, parent, start_hash):
           Your working copy is in mid-rebase. Either:
            * completely resolve like a normal git-rebase; OR
            * abort the rebase and mark this branch as dormant:
+                 git rebase --abort && \\
                  git config branch.%s.dormant true
 
           And then run `git rebase-update` again to resume.