Add git-bundle: move objects and references by archive

Add git-bundle: move objects and references by archive

Some workflows require use of repositories on machines that cannot be
connected, preventing use of git-fetch / git-push to transport objects and
references between the repositories.

git-bundle provides an alternate transport mechanism, effectively allowing
git-fetch and git-pull to operate using sneakernet transport. `git-bundle
create` allows the user to create a bundle containing one or more branches
or tags, but with specified basis assumed to exist on the target
repository. At the receiving end, git-bundle acts like git-fetch-pack,
allowing the user to invoke git-fetch or git-pull using the bundle file as
the URL. git-fetch and git-ls-remote determine they have a bundle URL by
checking that the URL points to a file, but are otherwise unchanged in
operation with bundles.

The original patch was done by Mark Levedahl <mdl123@verizon.net>.

It was updated to make git-bundle a builtin, and get rid of the tar
format: now, the first line is supposed to say "# v2 git bundle", the next
lines either contain a prerequisite ("-" followed by the hash of the
needed commit), or a ref (the hash of a commit, followed by the name of
the ref), and finally the pack. As a result, the bundle argument can be
"-" now.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>