Add unstage files feature

Add GitService.unstageFiles() using JGit reset to move files from
staged back to modified. Exposed via POST /repo/{name}/unstage
endpoint, repo page with checkboxes, and telnet menu option 7.
Includes unit tests for all layers.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

.gitignore

@@ -35,3 +35,6 @@ out/
 
 ### VS Code ###
 .vscode/
+
+# WebGit
+/webgit/