Upgrade libgit2 version

[rafeca]

This PR upgrades the version of libgit2 used to the commit libgit2/libgit2@9084712

This version contains the regression fix from libgit2/libgit2#4809 which prevented libgit2 to get upgraded and which we believe will make Atom builds less flaky.

The reason why I haven't upgraded to latest master version is because I cannot manage to build libgit2 with node-gyp with the changes in libgit2/libgit2#4919: when applying these changes I get the following error message from node-gyp:

In file included from ../deps/libgit2/src/hash/hash_generic.c:8:
../deps/libgit2/src/hash/hash_generic.h:24:17: error: static declaration of 'git_hash_global_init' follows non-static declaration
GIT_INLINE(int) git_hash_global_init(void)
                ^
../deps/libgit2/src/hash.h:34:5: note: previous declaration is here
int git_hash_global_init(void);

I've been able to work around the error by moving the static declaration in https://github.com/libgit2/libgit2/blob/master/src/hash/hash_generic.h#L24-L27 to L12, but I'm not exactly familiar with C so I have no idea if this is a good solution 😅 /cc @pks-t

Note: This is a big upgrade: It includes libgit2 changes from almost a year and 960 commits.

[rafeca]

Well this PR at the end seems to build fine 😓

If there's no opposition, tomorrow I'll merge it and release a new version of git-utils to try to integrate it as soon as possible into Atom.

I know that this is not ideal (since we're not pinning libgit2 to a stable version), but this will at least give us early signal about whether upgrading libgit2 fixes some of the Atom crashes.