scm: mercurial: split latest_changesets test to tag and branch

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extra : convert_revision : svn%3Ae93f8b46-1217-0410-a6f0-8f06a7374b81/trunk%4012748
scm: mercurial: use 12 chars id for format_changeset_identifier (#14361)

--HG--
extra : convert_revision : svn%3Ae93f8b46-1217-0410-a6f0-8f06a7374b81/trunk%4012749
scm: mercurial: add check scmid to RepositoryMercurialTest#test_fetch_changesets_from_scratch (#14361)

--HG--
extra : convert_revision : svn%3Ae93f8b46-1217-0410-a6f0-8f06a7374b81/trunk%4012750