Squashed 'deps/libgit2/' changes from 16e7596..9b87998

9b87998 Merge remote-tracking branch 'origin/cmn/update-zlib'
98ce231 Merge pull request #2461 from jacquesg/ssh2-warnings
ae241ae Include libssh2.h before git2.h (transport.h)
268dafa Fix git_cred_ssh_interactive_callback signature
b0ed61f Merge pull request #2460 from libgit2/cmn/sched-yield
905fb59 Move yield to the tests and enable for FreeBSD
193fe9c Merge pull request #2459 from libgit2/cmn/http-url-path
1380e7c netops: error out on url without a path
cb6e68c Merge pull request #2449 from libgit2/cmn/maint-21
c1bf294 Merge pull request #2455 from ethomson/equal_oid
4df4ebd Merge pull request #2453 from ethomson/checkout_index
b0ca1b1 Merge pull request #2452 from libgit2/cmn/clone-custom-repo
de3cf80 Merge pull request #2456 from libgit2/cmn/ssh-send-everything
0963716 ssh: libssh2_channel_write() behaves like send()
6812afa clone: remote git_clone_into{,_local} from the public API
d58a64e clone: add a callback for repository creation
967f5a7 git_checkout_index: checkout other indexes
9879fee revwalk::simplify test should test
0cee70e Introduce cl_assert_equal_oid
1453bd2 Merge pull request #2451 from libgit2/rb/round-up-pool-allocations
00b8c21 ssh: always declare the libssh2 types
5fa8cda Round up pool alloc sizes for alignment
dcdb850 Merge pull request #2440 from phkelley/transports
eac63e6 ssh: create the right callback signature based on build options
e6b0ae7 ssl: init only once without threads
1697cd6 Improvements to git_transport extensibility
b42ff7c zlib: disable warning 4142 on MSVC
4f8ac21 zlib: get rid of compress.c and uncompr.c
2bc7605 zlib: get rid of gz*
a918558 zlib: add a few missing defines
bb54fad Merge branch 'cmn/zlib-update' into cmn/update-zlib
7cead31 Merge branch 'cmn/zlib-128' into cmn/zlib-update
4ca2d7e Update zlib to 1.2.8

git-subtree-dir: deps/libgit2
git-subtree-split: 9b87998c97fedcf0c22cef28bb0839ecd68c1efd
Merge commit 'c3686856bc9aa97424fc96cb85b9485e9ad15df9' into jacquesg/update-libgit2
Raw.xs: libgit2 now requires the user to include libssh2
15-merge.t: Added missing state_cleanup call
Repository.xs: clone() changes

Added 'remote_create' callback
Removed 'ignore_cert_errors' and 'remote_name' options
10-clone.t: Updated tests
Repository.pm: Updated clone() documentation
MakeMaker.pm: Try and be smarter about compiler options

* (Try) to support ActivePerl builds
Changes: update