Log Message:
Update libgit2 to 0.25.1, it includes security problem.

For full changes, please refer CHANGESLOG.md file.

* libgit2 v0.24.6 and libgit2 v0.25.1, January 9th, 2017

Includes two fixes, one performs extra sanitization for some edge cases in
the Git Smart Protocol which can lead to attempting to parse outside of the
buffer.

The second fix affects the certificate check callback. It provides a valid
parameter to indicate whether the native cryptographic library considered
the certificate to be correct. This parameter is always 1/true before these
releases leading to a possible MITM.

This does not affect you if you do not use the custom certificate callback
or if you do not take this value into account. This does affect you if you
use pygit2 or git2go regardless of whether you specify a certificate check
callback.