git-unix lower bound fix

Author: codinuum

packages/diffast-git/diffast-git.0.1/opam

@@ -18,7 +18,7 @@ bug-reports: "https://github.com/codinuum/diffast/issues"
 depends: [
   "ocaml" {>= "4.14"}
   "dune" {>= "3.17"}
-  "git-unix"
+  "git-unix" {>= "3.17"}
   "diffast-misc" {= version}
   "vlt" {>= "0.2.4"}
   "odoc" {with-doc}