gotadmin dump is used to export (part of) the history of the repository; at the moment it only generates git bundles (which are pack files with a header) but support to generate a fast-import stream is planned. ok/tweaks stsp and jamsek

All the special cases are now being explained last.

improvements and ok stsp@

Intended mostly for the regress suite, so we'll be able to test also the ref-delta code paths. ok stsp@

The -h and -V/--version flags must be given before the command; with those flags the command is actually optional, but don't over-complicate the usage string with these nitpicks. ok/improvements stsp@

Similar to `git init -b`. Includes a change to `got import` behaviour such that "main" is no longer hardcoded by default; instead, we import to the branch resolved via the repository's HEAD reference unless `got import -b` is used, and only if HEAD cannot be resolved to a branch do we fallback to "main". includes fix plus ok from stsp@

Patch by Josiah Frentsos; ok kn@

patch by Josiah Frentsos