[Pkg-rust-maintainers] Bug#857068: Bump libgit2 version to upstream release 0.25.1

[Ximin Luo]

Source: libgit2
Version: 0.24.5-1
Followup-For: Bug #857068

Hey, this would be useful for the cargo package. (see #860116)

At the moment we have to embed libgit2 0.25.1 in the cargo source package, this
is not ideal for Debian especially given that there was already a CVE and it's
not obvious to the security team that they would also need to patch cargo.

X

-- System Information:
Debian Release: 9.0
  APT prefers testing
  APT policy: (990, 'testing'), (500, 'unstable-debug'), (500, 'testing-debug'), (500, 'buildd-unstable'), (300, 'unstable'), (100, 'experimental'), (1, 'experimental-debug')
Architecture: amd64 (x86_64)
Foreign Architectures: i386

Kernel: Linux 4.9.0-1-amd64 (SMP w/4 CPU cores)
Locale: LANG=en_GB.utf8, LC_CTYPE=en_GB.utf8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)