API call for `hg censor`
Finished original development that was stalled because of difficulties related to non-UTF8 file paths.
Closes #177 (closed)
This underlines in particular that it is not possible to censor content that is smaller than 15 bytes, and we know that hg censor
has problems with files large enough not to be inlined in their index, but these are independent from Heptapod, improvements should happen in Mercurial itself.