Add apply-patch, basic revert and cherry-pick functionality (#17902)

This code adds a simple endpoint to apply patches to repositories and
branches on gitea. This is then used along with the conflicting checking
code in #18004 to provide a basic implementation of cherry-pick revert.

Now because the buttons necessary for cherry-pick and revert have 
required us to create a dropdown next to the Browse Source button
I've also implemented Create Branch and Create Tag operations.

Fix #3880 
Fix #17986 

Signed-off-by: Andrew Thornton <art27@cantab.net>

services/repository/files/cherry_pick.go

@@ -0,0 +1,126 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package files
+
+import (
+	"context"
+	"fmt"
+	"strings"
+
+	"code.gitea.io/gitea/models"
+	repo_model "code.gitea.io/gitea/models/repo"
+	user_model "code.gitea.io/gitea/models/user"
+	"code.gitea.io/gitea/modules/git"
+	"code.gitea.io/gitea/modules/log"
+	"code.gitea.io/gitea/modules/structs"
+	"code.gitea.io/gitea/services/pull"
+)
+
+// CherryPick cherrypicks or reverts a commit to the given repository
+func CherryPick(ctx context.Context, repo *repo_model.Repository, doer *user_model.User, revert bool, opts *ApplyDiffPatchOptions) (*structs.FileResponse, error) {
+	if err := opts.Validate(ctx, repo, doer); err != nil {
+		return nil, err
+	}
+	message := strings.TrimSpace(opts.Message)
+
+	author, committer := GetAuthorAndCommitterUsers(opts.Author, opts.Committer, doer)
+
+	t, err := NewTemporaryUploadRepository(ctx, repo)
+	if err != nil {
+		log.Error("%v", err)
+	}
+	defer t.Close()
+	if err := t.Clone(opts.OldBranch); err != nil {
+		return nil, err
+	}
+	if err := t.SetDefaultIndex(); err != nil {
+		return nil, err
+	}
+
+	// Get the commit of the original branch
+	commit, err := t.GetBranchCommit(opts.OldBranch)
+	if err != nil {
+		return nil, err // Couldn't get a commit for the branch
+	}
+
+	// Assigned LastCommitID in opts if it hasn't been set
+	if opts.LastCommitID == "" {
+		opts.LastCommitID = commit.ID.String()
+	} else {
+		lastCommitID, err := t.gitRepo.ConvertToSHA1(opts.LastCommitID)
+		if err != nil {
+			return nil, fmt.Errorf("CherryPick: Invalid last commit ID: %v", err)
+		}
+		opts.LastCommitID = lastCommitID.String()
+		if commit.ID.String() != opts.LastCommitID {
+			return nil, models.ErrCommitIDDoesNotMatch{
+				GivenCommitID:   opts.LastCommitID,
+				CurrentCommitID: opts.LastCommitID,
+			}
+		}
+	}
+
+	commit, err = t.GetCommit(strings.TrimSpace(opts.Content))
+	if err != nil {
+		return nil, err
+	}
+	parent, err := commit.ParentID(0)
+	if err != nil {
+		parent = git.MustIDFromString(git.EmptyTreeSHA)
+	}
+
+	base, right := parent.String(), commit.ID.String()
+
+	if revert {
+		right, base = base, right
+	}
+
+	description := fmt.Sprintf("CherryPick %s onto %s", right, opts.OldBranch)
+	conflict, _, err := pull.AttemptThreeWayMerge(ctx,
+		t.basePath, t.gitRepo, base, opts.LastCommitID, right, description)
+	if err != nil {
+		return nil, fmt.Errorf("failed to three-way merge %s onto %s: %v", right, opts.OldBranch, err)
+	}
+
+	if conflict {
+		return nil, fmt.Errorf("failed to merge due to conflicts")
+	}
+
+	treeHash, err := t.WriteTree()
+	if err != nil {
+		// likely non-sensical tree due to merge conflicts...
+		return nil, err
+	}
+
+	// Now commit the tree
+	var commitHash string
+	if opts.Dates != nil {
+		commitHash, err = t.CommitTreeWithDate(author, committer, treeHash, message, opts.Signoff, opts.Dates.Author, opts.Dates.Committer)
+	} else {
+		commitHash, err = t.CommitTree(author, committer, treeHash, message, opts.Signoff)
+	}
+	if err != nil {
+		return nil, err
+	}
+
+	// Then push this tree to NewBranch
+	if err := t.Push(doer, commitHash, opts.NewBranch); err != nil {
+		return nil, err
+	}
+
+	commit, err = t.GetCommit(commitHash)
+	if err != nil {
+		return nil, err
+	}
+
+	fileCommitResponse, _ := GetFileCommitResponse(repo, commit) // ok if fails, then will be nil
+	verification := GetPayloadCommitVerification(commit)
+	fileResponse := &structs.FileResponse{
+		Commit:       fileCommitResponse,
+		Verification: verification,
+	}
+
+	return fileResponse, nil
+}