Security: update git2 to fix undefined behavior

[danielparks]

This updates git2 to fix two security advisories that do not seem to
affect git-status-vars.

RUSTSEC-2026-0183

When calling Remote::list() for a remote of a git repository, when
that remote does not advertise any references, git2 passes a null
pointer to the unsafe function slice::from_raw_parts(). Based on the
safety section documentation of function, data must be non-null even
for slices of length zero. Thus, the use of a null pointer leads to
undefined behavior.

RUSTSEC-2026-0184

When a Blame is created via Blame::blame_buffer(), and a
BlameHunk is retrieved, the pointers to the original author,
original committer, final author, and final committer may be null if
unavailable. The corresponding BlameHunk methods then create
Signatures based on null pointers; attempting to access the data of
the Signatures leads to dereferencing null pointers.