annex.addunlocked expressions

annex.addunlocked expressions

* annex.addunlocked can be set to an expression with the same format used by
  annex.largefiles, in case you want to default to unlocking some files but
  not others.
* annex.addunlocked can be configured by git-annex config.

Added a git-annex-matching-expression man page, broken out from
tips/largefiles.

A tricky consequence of this is that git-annex add --relaxed
honors annex.addunlocked, but an expression might want to know the size
or content of an url, which it's not going to download. I decided it was
better not to fail, and just dummy up some plausible data in that case.

Performance impact should be negligible. The global config is already
loaded for annex.largefiles. The expression only has to be parsed once,
and in the simple true/false case, it should not do any additional work
matching it.