convert: introduce hascommitfrommap sink method
Mercurial has stable revision identifiers and rollback and strip. Revisions
referenced in the shamap are thus not necessarily still present but we can
easily check for it.
Subversion do not have stable identifiers and no rollback or strip(?). We must
thus assume that all revisions referenced from a shamap still must be present.
This method is similar to hascommitforsplicemap but different ...
convert: rename sink hascommit to hascommitforsplicemap
The name 'hascommit' sounds like something generic ... but it might
also throw exceptions in specific cases and it is thus (apparently)
only useful for splicemap.