tree: allow retrieval of raw attributes

When a tool needs to recreate the tree object (for example an
interface to another VCS), it needs to use the raw attributes,
forgoing any normalization.

Multiple init

Make git_index_read() cope with external additions and removals of the index file

network cancellation improvements

commit: Introduce git_commit_message_raw()

HTTP: handle "relative" redirects

This commit adds cancellation for the push operation. This work consists of:

1) Support cancellation during push operation
    - During object counting phase
    - During network transfer phase
        - Propagate GIT_EUSER error code out to caller
2) Improve cancellation support during fetch
    - Handle cancellation request during network transfer phase
    - Clear error string when cancelled during indexing
3) Fix error handling in git_smart__download_pack

Cancellation during push is still only handled in the pack building and
network transfer stages of push (and not during packbuilding).

An alternative for 'clone' with options

There's no need for this to be a pointer to somewhere else.

Hopefully clear up what they're for.

Make the difference more explicit.

This wasremoved as part of the large culling a few commits ago.

This is a small thing that by itself doesn't quite justify making the
user use clone_into.

The removal of many options which lead to the direct usage of the
user's checkout options means we should make sure they remain const.

This used to be done via transport flags, which was removed in a
previous commit.

The basic clone function is there to make it easy to create a "normal"
clone. Remove a bunch of options that are about changing the remote's
configuration.

Unify the code bases.

Downloading all tags is part of what makes it a clone instead of
simply a fetch.

This allows you to set up the repository and remote as you which to
have them before performing the clone operation.

Move this one as well, letting us have a single way of setting the
callbacks for the remote, and removing fields from the clone options.

The text progress and update_tips callbacks are already part of the
struct, which was meant to unify the callback setup, but the download
one was left out.

Add git_transport_register, git_transport_unregister

Config subsection name should allow to have ']' and '\\' should allow to escape any characters