cubicweb-vcwiki #3775304 Use hglib instead of mercurial [validation pending]

Thus avoid the lazy import and the thread-related complications.

prioritynormal
typeenhancement
done in0.2.0
load0.200
load left0.000
closed by#be316f25b5ca Use hglib instead of mercurial (closes #3775304)
patchUse hglib instead of mercurial (closes #3775304) [applied]