Some Taskcluster jobs such as the decision task and toolchain builds
do not need the entire source tree checked out. Those jobs use mercurial
sparse profiles to only check out what is needed. Thunderbird builds
mostly use the same profiles as Firefox; these are the new ones that
are required with one-repo.
Note that currently, there are no sparse checkout profiles on any
Thunderbird repository. Adding these files will not affect current
operations.
Differential Revision: https://phabricator.services.mozilla.com/D110432
Rob Lemley