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
 	}