Link dynamically to libgit2

With libgit2 v0.22 released, we can expect its API and ABI to remain
stable when installed on the system.

Linking dynamically allows us to use the go tool alone to build and
install the package.

git.go

@@ -3,6 +3,7 @@ package git
 /*
 #include <git2.h>
 #include <git2/sys/openssl.h>
+#cgo pkg-config: libgit2
 */
 import "C"
 import (