Currently, the caller potentially prepares files, add them to the VCS,
and then calls the function to create the commit, does whatever it wants
to do with it, then undoes it.
With the ultimate goal of not doing local changes to the work tree to
act on that try commit (for a push), this change in API makes it so
that everything except what do do with the commit is handled by one
function, that now takes paths and file contents for the files that were
previously created by the caller.
Differential Revision: https://phabricator.services.mozilla.com/D214214
