Mark some symbols to be deprecated #minor (#698)

This change introduces the file deprecated.go, which contains any
constants, functions, and types that are slated to be deprecated in the
next major release.

These symbols are deprecated because they refer to old spellings in
pre-1.0 libgit2. This also makes the build be done with the
`-DDEPRECATE_HARD` flag to avoid regressions.

This, together with
[gorelease](https://godoc.org/golang.org/x/exp/cmd/gorelease)[1] should
make releases safer going forward.

1: More information about how that works at
   https://go.googlesource.com/exp/+/refs/heads/master/apidiff/README.md

1 files changed, 7 insertions, 6 deletions

diff --git a/submodule.go b/submodule.go
index 5be38d0..1b5ec60 100644
--- a/submodule.go
+++ b/submodule.go
@@ -106,20 +106,21 @@ func (c *SubmoduleCollection) Lookup(name string) (*Submodule, error) {
 	return newSubmoduleFromC(ptr, c.repo), nil
 }
 
-type SubmoduleCbk func(sub *Submodule, name string) int
+// SubmoduleCallback is a function that is called for every submodule found in SubmoduleCollection.Foreach.
+type SubmoduleCallback func(sub *Submodule, name string) int
 
-//export SubmoduleVisitor
-func SubmoduleVisitor(csub unsafe.Pointer, name *C.char, handle unsafe.Pointer) C.int {
+//export submoduleCallback
+func submoduleCallback(csub unsafe.Pointer, name *C.char, handle unsafe.Pointer) C.int {
 	sub := &Submodule{(*C.git_submodule)(csub), nil}
 
-	if callback, ok := pointerHandles.Get(handle).(SubmoduleCbk); ok {
+	if callback, ok := pointerHandles.Get(handle).(SubmoduleCallback); ok {
 		return (C.int)(callback(sub, C.GoString(name)))
 	} else {
 		panic("invalid submodule visitor callback")
 	}
 }
 
-func (c *SubmoduleCollection) Foreach(cbk SubmoduleCbk) error {
+func (c *SubmoduleCollection) Foreach(cbk SubmoduleCallback) error {
 	runtime.LockOSThread()
 	defer runtime.UnlockOSThread()
 
@@ -366,7 +367,7 @@ func populateSubmoduleUpdateOptions(ptr *C.git_submodule_update_options, opts *S
 		return nil
 	}
 
-	populateCheckoutOpts(&ptr.checkout_opts, opts.CheckoutOpts)
+	populateCheckoutOptions(&ptr.checkout_opts, opts.CheckoutOpts)
 	populateFetchOptions(&ptr.fetch_opts, opts.FetchOptions)
 
 	return nil