add verify git read func

vendor/gopkg.in/src-d/go-billy.v4/osfs/os_posix.go

@@ -0,0 +1,21 @@
+// +build !windows
+
+package osfs
+
+import (
+	"syscall"
+)
+
+func (f *file) Lock() error {
+	f.m.Lock()
+	defer f.m.Unlock()
+
+	return syscall.Flock(int(f.File.Fd()), syscall.LOCK_EX)
+}
+
+func (f *file) Unlock() error {
+	f.m.Lock()
+	defer f.m.Unlock()
+
+	return syscall.Flock(int(f.File.Fd()), syscall.LOCK_UN)
+}