merge: remove last traces of fastforward merging
bbc97d419b16 introduced fast-forward branch merging.
66ed92ed115a partly removed it again.
The fastforward variable would always be false if branchmerge was false, so the
fastforward variable makes no difference and we can remove it completely.
http://mercurial.selenic.com/bts/issue436
$ hg init
$ hg -v log -v
$ hg -v log -v x