Investigate feasibility of revision diff fragments specification for SWHID

Current SWHID specification enables to point to a specific fragment inside a source code file, for instance swh:1:cnt:4d99d2d18326621ccdd70f5ea66c2e2ac236ad8b;origin=https://gitorious.org/ocamlp3l/ocamlp3l_cvs.git;visit=swh:1:snp:d7f1b9eb7ccb596c2622c4780febaa02549830f9;anchor=swh:1:rev:2db189928c94d62a3b4757b3eec68f0a4d4113f0;path=/Examples/SimpleFarm/simplefarm.ml;lines=9-15.

In a similar manner, it would be of interest to point through SWHIDs to specific fragments of a source code diff that can be derived from a SWH revision (see example here).

That task is here to investigate if such a specification could be added to revision SWHIDs and if it can be resolved afterwards by the SWH Web application.

After a first discussion on the subject with @rdicosmo, this could be achieved by using the path and lines qualifiers on a revision SWHID.

Nervertheless, diffs may be complex and there is plenty of edge cases to handle. So let's see if we can cover all of them using the above qualifiers.

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Migrated from T2586 (view on Phabricator)