Unbreak with libgit2 >=1.9.0.

Index: modcargo-crates/libgit2-sys-0.17.0+1.8.1/build.rs
--- modcargo-crates/libgit2-sys-0.17.0+1.8.1/build.rs.orig
+++ modcargo-crates/libgit2-sys-0.17.0+1.8.1/build.rs
@@ -7,7 +7,7 @@ use std::process::Command;
 /// Tries to use system libgit2 and emits necessary build script instructions.
 fn try_system_libgit2() -> Result<pkg_config::Library, pkg_config::Error> {
     let mut cfg = pkg_config::Config::new();
-    match cfg.range_version("1.8.1".."1.9.0").probe("libgit2") {
+    match cfg.range_version("1.8.1".."2.0.0").probe("libgit2") {
         Ok(lib) => {
             for include in &lib.include_paths {
                 println!("cargo:root={}", include.display());
