repository: add a getter and remove function for git's prepared message

Add function to query for compile time settings.

The 'git revert/cherry-pick/merge -n' commands leave .git/MERGE_MSG
behind so that git-commit can find it. As we don't yet support these
operations, users who are shelling out to let git perform these
operations haven't had a convenient way to get this message.

These functions allow the user to retrieve the message and remove it
when she's created the commit.

git_config_find_* does not set a git error

travis: be more idiomatic with the environment

odb: allow creating an ODB backend from a packfile index

Unify the transport code

SSL isn't the only way that a transport can be encrypted. The new name
will make it easier to merge the SSH support.

Using the callbacks makes it clearer and reduces the amount of #ifdefs
and ifs and we need in the code.

Instad of each transport having its own function and logic to get to
its refs, store them directly in transport.

Leverage the new gitno_buffer to make the parsing and storing of the
refs use common code and get rid of the git_protocol struct.

This allows us to add capabilitites to both at the same time, keeps
them in sync and removes a lot of code.

gitno_buffer now uses a callback to fill its buffer, allowing us to
use the same interface for git and http (which uses callbacks).

For the transition, http is going to keep its own logic until the
git/common code catches up with the implied multi_ack that http
has. This also has the side-effect of making the code cleaner and more
correct regardingt he protocol.

git_oid_cmp: inline memcmp by hand to optimize

git.git uses an inlined hashcmp function instead of memcmp, since it
performes much better when comparing hashes (most hashes compared
diverge within the first byte).

Measurements and rationale for the curious reader:
http://thread.gmane.org/gmane.comp.version-control.git/172286

odb: add some documentation to the foreach() test

Add a struct for network callbacks

Yes, we can get the repository from the owner of the object, but having
it marked explicitly makes the API more consistent.

Add support for push-urls