GPG signed commits

[Rob Meng]

Not sure if these is a local config I can enable for this. I can't find any thing that enables signing on commit-tree automatically.

If there isn't a git config, I see two ways of adding this feature:

1. add a config line for commit signing and optionally allow keyid to be passed in from that config line. i.e.

None   => don't sign
""     => sign with default key
keyid  => sign with key id

2. check git config --list and use the key and behavior in there.

[Edward Z. Yang]

I would accept a patch that emulates git's porcelain behavior w.r.t commit.gpgsign