Fix single revision selection with hg >= 4.5
Fix single revision selection with hg >= 4.5

The behavior or 'hg log --follow-first -r <revset>' has changed in
mercurial 4.5: https://www.mercurial-scm.org/repo/hg/rev/a177c6aa055a

This cause 'asv run' with a single revision to resolve to all ancestors
of revision and following first parent instead of only the given single
revision.

Only use --follow-first only for get_branch_commits() which is needed to
draw the graph correctly in case of merges.

This fixes running tests with hg >= 4.5

pv/asv,airspeed-velocity/asv,spacetelescope/asv,spacetelescope/asv,qwhelan/asv,airspeed-velocity/asv,spacetelescope/asv,airspeed-velocity/asv,qwhelan/asv,qwhelan/asv,pv/asv,airspeed-velocity/asv,pv/asv,pv/asv,qwhelan/asv,spacetelescope/asv