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, 5 insertions, 7 deletions

diff --git a/reference.go b/reference.go
index 7b5e3c2..524d7bb 100644
--- a/reference.go
+++ b/reference.go
@@ -424,7 +424,7 @@ func (i *ReferenceIterator) Names() *ReferenceNameIterator {
 }
 
 // NextName retrieves the next reference name. If the iteration is over,
-// the returned error is git.ErrIterOver
+// the returned error code is git.ErrorCodeIterOver
 func (v *ReferenceNameIterator) Next() (string, error) {
 	var ptr *C.char
 
@@ -440,7 +440,7 @@ func (v *ReferenceNameIterator) Next() (string, error) {
 }
 
 // Next retrieves the next reference. If the iterationis over, the
-// returned error is git.ErrIterOver
+// returned error code is git.ErrorCodeIterOver
 func (v *ReferenceIterator) Next() (*Reference, error) {
 	var ptr *C.git_reference
 
@@ -470,7 +470,7 @@ func (v *ReferenceIterator) Free() {
 	C.git_reference_iterator_free(v.ptr)
 }
 
-// ReferenceIsValidName ensures the reference name is well-formed.
+// ReferenceIsValidName returns whether the reference name is well-formed.
 //
 // Valid reference names must follow one of two patterns:
 //
@@ -483,10 +483,8 @@ func (v *ReferenceIterator) Free() {
 func ReferenceIsValidName(name string) bool {
 	cname := C.CString(name)
 	defer C.free(unsafe.Pointer(cname))
-	if C.git_reference_is_valid_name(cname) == 1 {
-		return true
-	}
-	return false
+
+	return C.git_reference_is_valid_name(cname) == 1
 }
 
 const (