phabricator: don't assume the existence of properties of local:commits
Not all the properties are guaranteed to be there, so if we don't check first
we could die with a KeyError.
Differential Revision: https://phab.mercurial-scm.org/D6299
phabricator: include branch in the diffproperty metadata
This does not make Phabricator display the branch in web UI anywhere as that
still need us to use creatediff API for that. However a future patch will make
phabread use this to include the branch in its `hg import`-able output.
Differential Revision: https://phab.mercurial-scm.org/D6297