Windows compatibility fixes 

ssh: provide a factory function for setting ssh paths

Support refspecs with the asterisk in the middle

Properly report failure when expanding a packfile

merge: don't open COMMIT_MSG unless we need to append conflicts

Fixup git_attr_value's comment to be recognised as documentation, and
include the definitions needed for clang to parse reset.h such that
it shows up in the documentation.

This fixes #2430.

git allows you to set which paths to use for the git server programs
when connecting over ssh; and we want to provide something similar.

We do this by providing a factory function which can be set as the
remote's transport callback which will set the given paths upon
creation.

We used to assume a refspec would only have an asterisk in the middle of
their respective pattern. This has not been a valid assumption for some
time now with git.

Instead of assuming where the asterisk is going to be, change the logic
to treat each pattern as having two halves with a replacement bit in the
middle, where the asterisk is.

When transforming a non-pattern refspec, we simply need to copy over the
opposite string. Move that logic up to the wrapper so we can assume a
pattern refspec in the transformation function.

Fix SSH2 warnings

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.)