Phabricator deletes all branches in a mirror repository that are not in the Phabricator repository.
**Reproduction steps:**
* Create a new repository "hosted on phabricator"
* add an external (e.g. github) repository as mirror
* Create a branch `foo` and push it to the external repository
* Wait until phabricator updates the repository (or force it)
**Expected results:**
Phabricator should only mirror branches that are added in phabricator. So it should keep the `foo` branch as is.
**Current result:**
The `foo` branch in the external repository will get removed by phabracitor as it does not exist there.
I think there should be at least a configuration option to change this behavior.
A workaround is to configure phabricator to only the `master` branch. That keeps the external branches intact.