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, 1 insertions, 1 deletions

diff --git a/rebase.go b/rebase.go
index b7f88a8..f3746c1 100644
--- a/rebase.go
+++ b/rebase.go
@@ -129,7 +129,7 @@ type RebaseOptions struct {
 	InMemory              int
 	RewriteNotesRef       string
 	MergeOptions          MergeOptions
-	CheckoutOptions       CheckoutOpts
+	CheckoutOptions       CheckoutOptions
 	CommitSigningCallback CommitSigningCallback
 }