Alpha releases: How to tag

As we are approaching the first alpha release, let's think about how to tag
and label it and how to schedule those two operations with respect to one
another.

The typical process for, say, a beta release is

- apply version number change to source tree
- commit
- tag
(repeat for next beta)

The problem, arguably, with this is that the source tree would then read
8.5alpha1 until alpha2, instead of the 8.5devel that we are used to. The fix
could be:

- apply version number change to source tree
- commit
- tag
- revert version number change in source tree
- commit
(repeat for next alpha)

or more sophisticatedly

- branch
- apply version number change to source tree
- commit
- tag

or alternatively no tag at all, just apply version number and build tarball.

Comments?