HgLoaderFromDisk: Rename 'swhid' to 'sha1git' in variable names

SWHID has a specific meaning defined in https://docs.softwareheritage.org/devel/swh-model/persistent-identifiers.html
(in short, the sha1_git is only part of a SWHID), but this variables store
only the hash part of the SWHIDs.