With permission from Jelmer Vernooij to use my pull request config for Dulwich, and Jelmer's name, as a real-life example. ok tracey, jamsek

This adds a new flag, `got commit -A', that allows to specify the commit author but retains the "committer" (which is obtained as usual via GOT_AUTHOR, got.conf or git config.) with lots of help from and ok stsp

It doesn't really make sense to use mesg after a fold or drop, or after another mesg. it currently "works" as intended, but the behaviour is confusing and not useful, better abort the operation as it's probably not what the user intended. Suggested by and ok stsp@

prompted by a question by op@

This functionality is better suited for gotadmin because it is technically a server-side repository operation when we consider the 'got' tool as the client-side tool. I have plans to move 'got import' into gotadmin as well. ok op@

Existing configuration files will keep working for now since the old name with dashes is still recognized by the config parser.