tagcache: distinguish between invalid and missing entries The TortoiseHg repo has typically not had a newly applied tag accessible by name for recent releases, for unknown reasons. Deleting and rebuilding the tag cache doesn't fix it, though deleting the cache and running `hg log -r $new_tag` does. Eventually the situation does sort itself out for new clones from the server. In an effort to figure out what the issue is, Pierre-Yves David suggested listing these entries in the debug output more specifically. This isn't complete yet- the second test change that says "missing" is more like "invalid", since it was truncated. The problem there is the code that reads the raw array truncates any partial records and then fills it with 0xFF, which signifies that it is missing. As a side note, that means the check for the length when validating an existing entry never fails. Differential Revision: https://phab.mercurial-scm.org/D9811