Change git branch command line like CPython

vm/build.rs

@@ -23,7 +23,7 @@ fn git_tag() -> String {
 }
 
 fn git_branch() -> String {
-    git(&["rev-parse", "--abbrev-ref", "HEAD"])
+    git(&["name-rev", "--name-only", "HEAD"])
 }
 
 fn git(args: &[&str]) -> String {