Move yield to the tests and enable for FreeBSD

Move the definition of git_thread_yield() to the test which needs it and
add the correct definition for it for FreeBSD and derivatives.

Original patch adding FreeBSD and derivatives by @jacquesg.

netops: error out on url without a path

In order to connect to a remote server, we need to provide a path to the
repository we're interested in. Consider the lack of path in the url an
error.

Maint fixes for ssl initing and ssh exposure

Introduce `cl_assert_equal_oid`

git_checkout_index: checkout other indexes

Provide a callback to customize the repository on clone

ssh: libssh2_channel_write() behaves like send()

When the stream writing function was written, it assume that
libssh2_channel_write() would always write all of the data to the
wire. This is only true for the first 32k of data, which it tries to
fit into one ssh packet.

Since it can perform short writes, call it in a loop like we do for
send(), advancing the buffer offset.

As git_clone now has callbacks to configure the details of the
repository and remote, remove the lower-level functions from the public
API, as they lack some of the logic from git_clone proper.

Analogously to the remote creation callback, provide a way for the user
of git_clone() to create the repository with whichever options they
desire via callback.

git_checkout_index can now check out other git_index's (that are not
necessarily the repository index).  This allows checkout_index to use
the repository's index for stat cache information instead of the index
data being checked out.  git_merge and friends now check out their
indexes directly instead of trying to blend it into the running index.

The revwalk::simplify test was not actually tested the values from
the revwalk against the expected.  (Further, the expected had two
IDs transposed.)

Round up pool alloc sizes for alignment

This lets a user decide they do want to use keyboard-interactive after
they've compiled.

To make sure that items returned from pool allocations are aligned
on nice boundaries, this rounds up all pool allocation sizes to a
multiple of 8.  This adds a small amount of overhead to each item.

The rounding up could be made optional with an extra parameter to
the pool initialization that turned on rounding only for pools
where item alignment actually matters, but I think for the extra
code and complexity that would be involved, that it makes sense
just to burn a little bit of extra memory and enable this all the
time.

Improvements to git_transport extensibility

When linking against libssh2, create the transport.h such that it
contains its definition for custom crypto and keyboard-interactive
callbacks.

If we don't link against libssh2, create an equivalent signature which
has void pointers instead of pointers to libssh2 structures.

This would be one way to fix #2438.

The OpenSSL library-loading functions do not expect to be called
multiple times. Add a flag in the non-threaded libgit2 init so we only
call once.

This fixes #2446.

Fix assert when receiving uncommon sideband packet

git_remote_set_transport now takes a transport factory rather than a transport
git_clone_options now allows the caller to specify a remote creation callback

checkout::conflict tests: only test owner mode

The checkout::conflict type conflict tests were failing because
they were overly assertive about the resultant mode, testing
group & other bits, which failed miserably for people who had a
umask less restrictive than 022.  Only test the resultant owner bits.

Fixes #2443 Zero size arrays are an extension

Fix a couple of leaks

Set a message when we fail to lock.

Also make the put function void, since it's called from free, which
cannot report errors. The only errors we can experience here are
internal state corruption, so we assert that we are trying to put a
pack which we have previously got.

As reported by coverity, we would leak some memory in error conditions.

If we fail to insert the packfile in the map, make sure to free it.

This makes the free function only attempt to remove its mwindows from
the global list if we have opened the packfile to avoid accessing the
list unlocked.

We need to tell Travis to build the master branch or it won't build it
or the pull requests.

Update text=auto / core.autocrlf=false behavior

Export git_revert_commit