For instance, if the bug-prefix is set to "b/", git cl upload will offer a
line with "Bug: b/", however it won't get stripped away if it's left blank,
unlike the default "Bug:" line. This change fixes that inconsistency by
also taking the bug-prefix into account when stripping.

Change-Id: Ib6e4d18c1ff52ec77cd1422be15b1e6920332238
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1528972Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Jonas Termansen <sortie@google.com>