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@

Additional testing by Thomas Adam. ok tracey

Known to work against git-daemon and github Git server implementations. Tests by abieber, naddy, jrick, and myself. Man page additions reviewed by Lucas.

Previous default output remains when test cases are run individually. ok tracey

ok stsp