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>

modules/git/diff.go

@@ -32,6 +32,21 @@ func GetRawDiff(ctx context.Context, repoPath, commitID string, diffType RawDiff
 	return GetRawDiffForFile(ctx, repoPath, "", commitID, diffType, "", writer)
 }
 
+// GetReverseRawDiff dumps the reverse diff results of repository in given commit ID to io.Writer.
+func GetReverseRawDiff(ctx context.Context, repoPath, commitID string, writer io.Writer) error {
+	stderr := new(bytes.Buffer)
+	cmd := NewCommand(ctx, "show", "--pretty=format:revert %H%n", "-R", commitID)
+	if err := cmd.RunWithContext(&RunContext{
+		Timeout: -1,
+		Dir:     repoPath,
+		Stdout:  writer,
+		Stderr:  stderr,
+	}); err != nil {
+		return fmt.Errorf("Run: %v - %s", err, stderr)
+	}
+	return nil
+}
+
 // GetRawDiffForFile dumps diff results of file in given commit ID to io.Writer.
 func GetRawDiffForFile(ctx context.Context, repoPath, startCommit, endCommit string, diffType RawDiffType, file string, writer io.Writer) error {
 	repo, closer, err := RepositoryFromContextOrOpen(ctx, repoPath)
@@ -221,8 +236,7 @@ func CutDiffAroundLine(originalDiff io.Reader, line int64, old bool, numbersOfLi
 			}
 		}
 	}
-	err := scanner.Err()
-	if err != nil {
+	if err := scanner.Err(); err != nil {
 		return "", err
 	}
 

modules/structs/repo_file.go

@@ -50,6 +50,14 @@ type UpdateFileOptions struct {
 	FromPath string `json:"from_path" binding:"MaxSize(500)"`
 }
 
+// ApplyDiffPatchFileOptions options for applying a diff patch
+// Note: `author` and `committer` are optional (if only one is given, it will be used for the other, otherwise the authenticated user will be used)
+type ApplyDiffPatchFileOptions struct {
+	DeleteFileOptions
+	// required: true
+	Content string `json:"content"`
+}
+
 // FileLinksResponse contains the links for a repo's file
 type FileLinksResponse struct {
 	Self    *string `json:"self"`