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>

src/main/resources/templates/repo.html

@@ -55,14 +55,20 @@ New branch: <input type="text" name="branch" size="20">
 <p th:if="${#lists.isEmpty(modifiedFiles)}"><i>No modified files.</i></p>
 
 <h3>Staged Files</h3>
-<table border="1" cellpadding="4" cellspacing="0" th:if="${!#lists.isEmpty(stagedFiles)}">
+<form method="post" th:action="@{/repo/{name}/unstage(name=${name})}" th:if="${!#lists.isEmpty(stagedFiles)}">
+<table border="1" cellpadding="4" cellspacing="0">
 <tr>
+<th>Unstage</th>
 <th>File</th>
 </tr>
 <tr th:each="file : ${stagedFiles}">
+<td><input type="checkbox" name="files" th:value="${file}"></td>
 <td th:text="${file}"></td>
 </tr>
 </table>
+<br>
+<input type="submit" value="Unstage Selected">
+</form>
 <p th:if="${#lists.isEmpty(stagedFiles)}"><i>No staged files.</i></p>
 
 <form method="post" th:action="@{/repo/{name}/commit(name=${name})}" th:if="${!#lists.isEmpty(stagedFiles)}">