Users sometimes type in "y" to confirm the default patchset message,
resulting in a patchset being uploaded with "y" as the message.

We can make a fair assumption that no-one would willingly use a patchset
message of "y", so this CL uses the default patchset message whenever
this happens.

(I have personally done this myself and have seen at least one other
person do the same.)

Bug: None
Change-Id: Iad314d35aa830b62152ab2910eb37d876b7e450b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2861643Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Commit-Queue: Michael Cui <mlcui@google.com>