annex.addunlocked support for tree imports

annex.addunlocked support for tree imports

Honor annex.addunlocked configuration when importing a tree from a special
remote.

Note, in a --no-content import, the object file will not be populated
(usually) and so expressions that match on mime type will not match. Tested
this and it works ok, the file just ends up locked. Updated docs for the
mime expressions to mention that they can't match when the file is present

Note that in Command.Sync.pullThirdPartyPopulated, recordImportTree is
called without a AddUnlockedMatcher. Since the tree generated here is not
exposed to the user and does not contain usual filenames, there is no need
of the overhead of checking it.