*: Localize code that uses git2go in one small place

As will be explained in the next patch git2go exposes a lot of unsafety
to the users that can really lead to crashes and data corruption. We are
going to fix that via "always safe" wrapper over git2go.

-> Factor all code, that uses git2go, into internal/git package as a
   preparatory step for that.

No semantic change for now.

/reviewed-on !12

git-backup.go

@@ -90,7 +90,7 @@ import (
 	"lab.nexedi.com/kirr/go123/xstrings"
 	"lab.nexedi.com/kirr/go123/xsync"
 
-	git "github.com/libgit2/git2go/v31"
+	"lab.nexedi.com/kirr/git-backup/internal/git"
 )
 
 // verbose output

git-backup_test.go

@@ -36,7 +36,7 @@ import (
 	"lab.nexedi.com/kirr/go123/xruntime"
 	"lab.nexedi.com/kirr/go123/xstrings"
 
-	git "github.com/libgit2/git2go/v31"
+	"lab.nexedi.com/kirr/git-backup/internal/git"
 )
 
 func xgetcwd(t *testing.T) string {

gitobjects.go

-// Copyright (C) 2015-2021  Nexedi SA and Contributors.
+// Copyright (C) 2015-2025  Nexedi SA and Contributors.
 //                          Kirill Smelkov <kirr@nexedi.com>
 //
 // This program is free software: you can Use, Study, Modify and Redistribute
@@ -33,7 +33,7 @@ import (
 	"lab.nexedi.com/kirr/go123/mem"
 	"lab.nexedi.com/kirr/go123/xstrings"
 
-	git "github.com/libgit2/git2go/v31"
+	"lab.nexedi.com/kirr/git-backup/internal/git"
 )
 
 // read/write raw objects

internal/git/git.go

+// Copyright (C) 2025  Nexedi SA and Contributors.
+//                     Kirill Smelkov <kirr@nexedi.com>
+//
+// This program is free software: you can Use, Study, Modify and Redistribute
+// it under the terms of the GNU General Public License version 3, or (at your
+// option) any later version, as published by the Free Software Foundation.
+//
+// You can also Link and Combine this program with other software covered by
+// the terms of any of the Free Software licenses or any of the Open Source
+// Initiative approved licenses and Convey the resulting work. Corresponding
+// source of such a combination shall include the source code for all other
+// software used.
+//
+// This program is distributed WITHOUT ANY WARRANTY; without even the implied
+// warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+//
+// See COPYING file for full licensing terms.
+// See https://www.nexedi.com/licensing for rationale and options.
+
+// Package internal/git wraps package git2go.
+package git
+
+import (
+	git2go "github.com/libgit2/git2go/v31"
+)
+
+const (
+	ObjectAny     = git2go.ObjectAny
+	ObjectInvalid = git2go.ObjectInvalid
+	ObjectCommit  = git2go.ObjectCommit
+	ObjectTree    = git2go.ObjectTree
+	ObjectBlob    = git2go.ObjectBlob
+	ObjectTag     = git2go.ObjectTag
+)
+
+
+type (
+	ObjectType = git2go.ObjectType
+	Oid        = git2go.Oid
+	Signature  = git2go.Signature
+	TreeEntry  = git2go.TreeEntry
+)
+
+
+type Repository struct {
+	repo       *git2go.Repository
+	References *ReferenceCollection
+}
+
+type ReferenceCollection struct {
+	r *Repository
+}
+
+type Reference struct {
+	ref *git2go.Reference
+}
+
+type Commit struct {
+	commit *git2go.Commit
+}
+
+type Tree struct {
+	tree *git2go.Tree
+}
+
+type Odb struct {
+	odb *git2go.Odb
+}
+
+type OdbObject struct {
+	obj *git2go.OdbObject
+}
+
+
+// function and methods to navigate object hierarchy from Repository to e.g. OdbObject or Commit.
+
+func OpenRepository(path string) (*Repository, error) {
+	repo, err := git2go.OpenRepository(path)
+	if err != nil {
+		return nil, err
+	}
+	r := &Repository{repo: repo}
+	r.References = &ReferenceCollection{r}
+	return r, nil
+}
+
+func (rdb *ReferenceCollection) Create(name string, id *Oid, force bool, msg string) (*Reference, error) {
+	ref, err := rdb.r.repo.References.Create(name, id, force, msg)
+	if err != nil {
+		return nil, err
+	}
+	return &Reference{ref}, nil
+}
+
+func (r *Repository) LookupCommit(id *Oid) (*Commit, error) {
+	commit, err := r.repo.LookupCommit(id)
+	if err != nil {
+		return nil, err
+	}
+	return &Commit{commit}, nil
+}
+
+func (c *Commit) Tree() (*Tree, error) {
+	tree, err := c.commit.Tree()
+	if err != nil {
+		return nil, err
+	}
+	return &Tree{tree}, nil
+}
+
+func (r *Repository) Odb() (*Odb, error) {
+	odb, err := r.repo.Odb()
+	if err != nil {
+		return nil, err
+	}
+	return &Odb{odb}, nil
+}
+
+func (o *Odb) Read(oid *Oid) (*OdbObject, error) {
+	obj, err := o.odb.Read(oid)
+	if err != nil {
+		return nil, err
+	}
+	return &OdbObject{obj}, nil
+}
+
+
+// wrappers over methods
+
+func (c *Commit) ParentCount() uint	{ return c.commit.ParentCount() }
+func (o *OdbObject) Type() ObjectType	{ return o.obj.Type() }
+
+
+func (r *Repository) Path() string {
+	return r.repo.Path()
+}
+
+func (r *Repository) DefaultSignature() (*Signature, error) {
+	return r.repo.DefaultSignature()
+}
+
+
+func (c *Commit) Message() string {
+	return c.commit.Message()
+}
+
+func (c *Commit) ParentId(n uint) *Oid {
+	return c.commit.ParentId(n)
+}
+
+func (t *Tree) EntryByName(filename string) *TreeEntry {
+	return t.tree.EntryByName(filename)
+}
+
+
+func (o *Odb) Write(data []byte, otype ObjectType) (*Oid, error) {
+	return o.odb.Write(data, otype)
+}
+
+
+func (o *OdbObject) Id() *Oid {
+	return o.obj.Id()
+}
+
+func (o *OdbObject) Data() []byte {
+	return o.obj.Data()
+}

sha1.go

-// Copyright (C) 2015-2021  Nexedi SA and Contributors.
+// Copyright (C) 2015-2025  Nexedi SA and Contributors.
 //                          Kirill Smelkov <kirr@nexedi.com>
 //
 // This program is free software: you can Use, Study, Modify and Redistribute
@@ -27,7 +27,7 @@ import (
 
 	"lab.nexedi.com/kirr/go123/mem"
 
-	git "github.com/libgit2/git2go/v31"
+	"lab.nexedi.com/kirr/git-backup/internal/git"
 )
 
 const SHA1_RAWSIZE = 20
@@ -93,7 +93,7 @@ func (p BySha1) Len() int           { return len(p) }
 func (p BySha1) Swap(i, j int)      { p[i], p[j] = p[j], p[i] }
 func (p BySha1) Less(i, j int) bool { return bytes.Compare(p[i].sha1[:], p[j].sha1[:]) < 0 }
 
-// interoperability with git2go
+// interoperability with git package
 func (sha1 *Sha1) AsOid() *git.Oid {
 	return (*git.Oid)(&sha1.sha1)
 }