Require latest libgit2 to pull in bugfixes

This'll pull in a fix for #8258

Author: alexcrichton

Cargo.toml

@@ -44,7 +44,7 @@ jobserver = "0.1.21"
 lazycell = "1.2.0"
 libc = "0.2"
 log = "0.4.6"
-libgit2-sys = "0.12.5"
+libgit2-sys = "0.12.7"
 memchr = "2.1.3"
 num_cpus = "1.0"
 opener = "0.4"