git: prohibit push to remote named "git"

Since I'm going to make git::push_branches() update the repo view internally,
it should fail fast if the remote name is reserved. Before, the problem was
detected on git::import_refs().

cli/tests/test_git_push.rs

@@ -663,3 +663,23 @@ fn test_git_push_conflicting_branches() {
       Move branch branch1 from fd1d63e031ea to 8263cf992d33
     "###);
 }
+
+#[test]
+fn test_git_push_to_remote_named_git() {
+    let (test_env, workspace_root) = set_up();
+    let git_repo = {
+        let mut git_repo_path = workspace_root.clone();
+        git_repo_path.extend([".jj", "repo", "store", "git"]);
+        git2::Repository::open(&git_repo_path).unwrap()
+    };
+    git_repo.remote_rename("origin", "git").unwrap();
+
+    let stderr =
+        test_env.jj_cmd_failure(&workspace_root, &["git", "push", "--all", "--remote=git"]);
+    insta::assert_snapshot!(stderr, @r###"
+    Branch changes to push to git:
+      Add branch branch1 to 45a3aa29e907
+      Add branch branch2 to 8476341eb395
+    Error: Git remote named 'git' is reserved for local Git repository
+    "###);
+}

lib/src/git.rs

@@ -1064,6 +1064,11 @@ pub fn fetch(
 pub enum GitPushError {
     #[error("No git remote named '{0}'")]
     NoSuchRemote(String),
+    #[error(
+        "Git remote named '{name}' is reserved for local Git repository",
+        name = REMOTE_NAME_FOR_LOCAL_GIT_REPO
+    )]
+    RemoteReservedForLocalGitRepo,
     #[error("Push is not fast-forwardable")]
     NotFastForward,
     #[error("Remote rejected the update of some refs (do you have permission to push to {0:?}?)")]
@@ -1167,6 +1172,9 @@ fn push_refs(
     refspecs: &[String],
     callbacks: RemoteCallbacks<'_>,
 ) -> Result<(), GitPushError> {
+    if remote_name == REMOTE_NAME_FOR_LOCAL_GIT_REPO {
+        return Err(GitPushError::RemoteReservedForLocalGitRepo);
+    }
     let mut remote = git_repo.find_remote(remote_name).map_err(|err| {
         if is_remote_not_found_err(&err) {
             GitPushError::NoSuchRemote(remote_name.to_string())