merge_to_branch.py can fail partway through for many reasons (the EDITOR
environment variable not being set being one) and on the next run this
leads to an error saying:

    Exception: A merge is already in progress

It is not obvious to those doing their first merge how to get past this.
Searching the source code leads to the -f option but it should be
possible to proceed without searching the source. This change adds
"Use -f to continue" to the message.

Change-Id: Ic9d8e404e044be3308e5ae3ef3a4430e4aa3ccc2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1837028Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64096}