cubicweb-vcsfile #2735031 make sure revision changeset are not interpreted as anything else [validation pending]

for now, if one has an (weird) hg repo in which a branch is named as a revision changeset, the import mecanism affect the revision number of the nrahch head instead of the revision number of the changeset.

priorityimportant
typebug
done in1.12.3
load0.250
load left0.000
closed by#8e27ddad1f82 [import/mercurial] force mercurial to interpret changeset revision as such, #2a8cf2ecedef [import/mercurial] force mercurial to interpret changeset revision as such (closes #2735031)
patch[import/mercurial] force mercurial to interpret changeset revision as such (closes #2735031) [applied]