They tend to produce unwanted effects when an mIRC code is directly
followed by a text digit (which is going to happen often in this context, e.g.
with git hex commit IDs). the mIRC codes have no end delimiter, so
unless the client's recognizer is very carefully crafted it will eat
following text.

Not quite the same as laurentb's design; this one passes all the
commit and extractor data as a single JSON object, and expect to get
back same.

With manpage documentation including example script.
Also add the full e-mail to the available metadata (only git for now).

Signed-off-by: Eric S. Raymond <esr@thyrsus.com>

This is required in order to correctly handle pushes of non-default
branches to a bare repo.