Squashed 'deps/libgit2/' changes from 3a495c1..73dd663

73dd663 Merge pull request #2565 from linquize/typo
903a761 Merge pull request #2561 from jacquesg/merge-skip
52e9120 attr: Cleanup the containing dir logic
6259576 Merge pull request #2566 from libgit2/rb/attr-with-bare
1fbeb2f Fix attribute lookup in index for bare repos
2c22193 Fix typo
910cd2d Merge pull request #2563 from alexcrichton/rust-bindings
b491370 Add Rust to the language bindings list
a7fcac1 Merge pull request #2554 from linquize/fetch-head-tag
dc68ee8 Remove local unused index_repo variable
a565f36 Only check for workdir conflicts if the index has merged files
493c340 Added test case to illustrate bogus conflicts detected if no files were merged, and untracked files exist in the workdir.
d908351 When auto follow tags, FETCH_HEAD should list only newly followed tags

git-subtree-dir: deps/libgit2
git-subtree-split: 73dd663876bbc070bdc0c062ba3e14448ed11c30
Merge commit '9074c7f0812669015a72bc15c39f640a3d0dc4ac' into jacquesg/update-libgit2
Changes: update
Raw.xs: Let $a or $b be undef if the reference was created or removed
Changes: update