Update zlib to 1.2.8
Merge branch 'cmn/zlib-128' into cmn/zlib-update

Conflicts:
	deps/zlib/crc32.c
	deps/zlib/crc32.h
	deps/zlib/zconf.h
Merge branch 'cmn/zlib-update' into cmn/update-zlib
zlib: add a few missing defines
zlib: get rid of gz*
zlib: get rid of compress.c and uncompr.c
zlib: disable warning 4142 on MSVC

This is about benign redefinition of types. We're not interested in it.
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.
Merge pull request #2459 from libgit2/cmn/http-url-path

netops: error out on url without a path
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.
Merge pull request #2460 from libgit2/cmn/sched-yield

Move yield to the tests and enable for FreeBSD
Fix git_cred_ssh_interactive_callback signature
Include libssh2.h before git2.h (transport.h)
Merge pull request #2461 from jacquesg/ssh2-warnings

Fix SSH2 warnings
Merge remote-tracking branch 'origin/cmn/update-zlib'
Documentation fixes

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