Squashed 'deps/libgit2/' changes from 9e49cb7..e0383fa

e0383fa Merge pull request #2609 from linquize/describe-opts
c6e2621 Merge pull request #2615 from ethomson/mount_points
c0c8570 Merge pull request #2616 from ethomson/index_crlf
6a26488 Don't copy buffer in checkout unless needed
5e2cf2c Ensure filters (i.e. CRLF) are applied when checking out conflict content
8d3b2ee Introduce failing test for conflict filtering in index
24deacb Merge pull request #2618 from csware/msvc-no-z-prefix
969b6a4 is_empty_dir (wi32): cope with empty mount points
8d45b46 p_lstat win32: don't canonicalize volume mounts
cf1013a There is no "z" size specifier on MSVC
59186d9 describe: Initialize options for git_describe_format() if null
0494a7c describe: Do not crash if pass null option to git_describe_commit()

git-subtree-dir: deps/libgit2
git-subtree-split: e0383fa35f981c656043976a43c61bff059cb709
Merge commit '16bf710537c801f9ffb7893f96a055cad76fd3e7' into jacquesg/update-libgit2
Changes: update