devel/libgit2: update to 1.5.2

Changelog:
This is a bugfix release to improve the SSH key handling functionality that was introduced in v1.5.1.

libgit2 can now read the known_hosts file on Windows.
libgit2 will now examine all keys for a particular host in the known_hosts file for matches, to support remote hosts with multiple key types.

All users of the v1.5 release line are recommended to upgrade.

https://github.com/libgit2/libgit2/blob/v1.5.2/docs/changelog.md