* Removes mingw-compat.h
* Cleans up separation of compiler/platform idiosyncrasies
* Unifies mingw/msvc stat structures and functions
* (Tries to) hide more compiler specific implementation details (even in our internal API)

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