fetch-pack: start multi-head pulling.

fetch-pack: start multi-head pulling.

This is a beginning of resurrecting the multi-head pulling support
for git-fetch-pack command.  The git-fetch-script wrapper still
only knows about fetching a single head, without renaming, so it is
not very useful unless you directly call git-fetch-pack itself yet.

It also fixes a longstanding obsolete description of how the command
discovers the list of local commits.