Switch 'master' to 'main'

[mattecapu]

Since Git(Hub) changed the default branch name to main zulip-archive should use that when commiting new changes, I believe here and in subsequent lines.

Even better would be to allow the user to specify a preferred branch name.

[rht]

Right. Here is the thread of current discussion: https://chat.zulip.org/#narrow/stream/2-general/topic/master-slave.20term.