Create local 'sh fn which acts like c.j.shell/sh but turns Files into strings.
Initial :box-add functionallity.
Don't trust git log filename filtering. Filter commit paths ourselves.
Strip "tag:" prefixes off of refs in 'parse-sha-refs.
Fix issue in 'parse-tag where an unparsed tag would return "" instead of nil.

The problem with returning "" instead of nil is that "" matches
projects at the root (which have a path of "") when they should not
match at all.