git::Worktree::drop() now force-drops the worktree

This avoids leaving prunable worktrees around if we dirtied the worktree
and didn't commit.

This can happen in the following situation:

1. User runs `ent new`.

2. ent creates a new directory for the issue.

3. ent opens an editor to let the user type in the description of the
   new issue.  The editor saves to `ISSUE/description`.

4. User changes their mind and no longer wants to make a new issue, so
   they save an empty buffer and exit the editor.

5. ent sees that the file is empty, and returns an error from
   Issue::edit_description().

6. ent propagates the error up through program exit, and eventually
   the git::Worktree struct is dropped.  Since the worktree is dirty
   (it has the new issue dir with an empty description file in it),
   `git worktree remove` fails.

But `git worktree remove --force` works!

src/git.rs

@@ -20,9 +20,24 @@ pub struct Worktree {
 
 impl Drop for Worktree {
     fn drop(&mut self) {
-        let _result = std::process::Command::new("git")
+        let result = std::process::Command::new("git")
-            .args(["worktree", "remove", &self.path.path().to_string_lossy()])
+            .args([
+                "worktree",
+                "remove",
+                "--force",
+                &self.path.path().to_string_lossy(),
+            ])
             .output();
+        match result {
+            Err(e) => {
+                println!("failed to run git: {:#?}", e);
+            }
+            Ok(result) => {
+                if !result.status.success() {
+                    println!("failed to remove git worktree: {:#?}", result);
+                }
+            }
+        }
     }
 }