Add error message when cargo fix on an empty repo

Signed-off-by: hi-rustin <rustin.liu@gmail.com>

Author: 0xPoe

src/cargo/ops/fix.rs

@@ -154,6 +154,14 @@ fn check_version_control(config: &Config, opts: &FixOptions) -> CargoResult<()>
     if let Ok(repo) = git2::Repository::discover(config.cwd()) {
         let mut repo_opts = git2::StatusOptions::new();
         repo_opts.include_ignored(false);
+        if repo.is_empty()? {
+            bail!(
+                "no commits found in the git repository, and \
+                `cargo fix` can potentially perform destructive changes; if you'd \
+                like to suppress this error pass `--allow-dirty`, `--allow-staged`, \
+                or commit your changes"
+            )
+        }
         for status in repo.statuses(Some(&mut repo_opts))?.iter() {
             if let Some(path) = status.path() {
                 match status.status() {