Issue 256: fix parsing branch name from changelog
To retrieve a branch of an changeset, `stx:libscm` uses `{branch}`
branch keyword and then parses it as "name list". However, according
to documentation it is a single string:
branch String. The name of the branch on which the changeset was
committed.
This obviously caused problems when branch name had spaces in it.
This commit fixes the problem.
One remaining thing is that `stx:libscm` technically allows a changeset
to be in more than one branch which seems to be impossible in
Mercurial itself. This should be investigated and fixed, eventually.