Fix plain text joining to trim whitespace

Use appendWithSpace with trimmed text to avoid double spaces when joining
lines with leading whitespace.

Co-authored-by: alexr00 <38270282+alexr00@users.noreply.github.com>

Author: Copilot

src/github/folderRepositoryManager.ts

@@ -3356,7 +3356,7 @@ function unwrapCommitMessageBody(body: string): string {
 			}
 
 			// Join this line with a space
-			joinedLine += ' ' + nextLine;
+			joinedLine = appendWithSpace(joinedLine, nextLine.trim());
 			i++;
 		}