fix: Rename git_commit_nth_gen_ancestor's first argument

`git_commit_nth_gen_ancestor`'s first argument has been changed to
`ancestor` from the previous `commit` to bring it closer to the actual
definition (git2/commit.h line 282) and because the previous name is
also used for the second argument.

Author: tecc

libgit2-sys/lib.rs

@@ -2991,7 +2991,7 @@ extern "C" {
     pub fn git_commit_message_encoding(commit: *const git_commit) -> *const c_char;
     pub fn git_commit_message_raw(commit: *const git_commit) -> *const c_char;
     pub fn git_commit_nth_gen_ancestor(
-        commit: *mut *mut git_commit,
+        ancestor: *mut *mut git_commit,
         commit: *const git_commit,
         n: c_uint,
     ) -> c_int;