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, 63 insertions, 0 deletions

diff --git a/errorclass_string.go b/errorclass_string.go
new file mode 100644
index 0000000..1223442
--- /dev/null
+++ b/errorclass_string.go
@@ -0,0 +1,63 @@
+// Code generated by "stringer -type ErrorClass -trimprefix ErrorClass -tags static"; DO NOT EDIT.
+
+package git
+
+import "strconv"
+
+func _() {
+	// An "invalid array index" compiler error signifies that the constant values have changed.
+	// Re-run the stringer command to generate them again.
+	var x [1]struct{}
+	_ = x[ErrorClassNone-0]
+	_ = x[ErrorClassNoMemory-1]
+	_ = x[ErrorClassOS-2]
+	_ = x[ErrorClassInvalid-3]
+	_ = x[ErrorClassReference-4]
+	_ = x[ErrorClassZlib-5]
+	_ = x[ErrorClassRepository-6]
+	_ = x[ErrorClassConfig-7]
+	_ = x[ErrorClassRegex-8]
+	_ = x[ErrorClassOdb-9]
+	_ = x[ErrorClassIndex-10]
+	_ = x[ErrorClassObject-11]
+	_ = x[ErrorClassNet-12]
+	_ = x[ErrorClassTag-13]
+	_ = x[ErrorClassTree-14]
+	_ = x[ErrorClassIndexer-15]
+	_ = x[ErrorClassSSL-16]
+	_ = x[ErrorClassSubmodule-17]
+	_ = x[ErrorClassThread-18]
+	_ = x[ErrorClassStash-19]
+	_ = x[ErrorClassCheckout-20]
+	_ = x[ErrorClassFetchHead-21]
+	_ = x[ErrorClassMerge-22]
+	_ = x[ErrorClassSSH-23]
+	_ = x[ErrorClassFilter-24]
+	_ = x[ErrorClassRevert-25]
+	_ = x[ErrorClassCallback-26]
+	_ = x[ErrorClassRebase-29]
+	_ = x[ErrorClassPatch-31]
+}
+
+const (
+	_ErrorClass_name_0 = "NoneNoMemoryOSInvalidReferenceZlibRepositoryConfigRegexOdbIndexObjectNetTagTreeIndexerSSLSubmoduleThreadStashCheckoutFetchHeadMergeSSHFilterRevertCallback"
+	_ErrorClass_name_1 = "Rebase"
+	_ErrorClass_name_2 = "Patch"
+)
+
+var (
+	_ErrorClass_index_0 = [...]uint8{0, 4, 12, 14, 21, 30, 34, 44, 50, 55, 58, 63, 69, 72, 75, 79, 86, 89, 98, 104, 109, 117, 126, 131, 134, 140, 146, 154}
+)
+
+func (i ErrorClass) String() string {
+	switch {
+	case 0 <= i && i <= 26:
+		return _ErrorClass_name_0[_ErrorClass_index_0[i]:_ErrorClass_index_0[i+1]]
+	case i == 29:
+		return _ErrorClass_name_1
+	case i == 31:
+		return _ErrorClass_name_2
+	default:
+		return "ErrorClass(" + strconv.FormatInt(int64(i), 10) + ")"
+	}
+}