Add mainline to the list of HEAD branch names
Per GNOME/geary#324 (closed), Geary is renaming its default branch to mainline
. This adds that to d-l's list of known HEAD branch names. It's not last, since during the transition, both master
and mainline
are present.
It would be more robust to actually just ask git which branch HEAD points to, but that seems to be a bit of a chicken and egg problem with the model, as-is.