Put worktree outside of repo in manual steps

If a backport fails then steps are given to create a backport manually.
Currently, these steps put the worktree inside the repository folder,
which causes git to see the worktree as untracked files. This change
puts the worktree outside of the repository to avoid this problem.

Signed-off-by: Andrew Ross <andrross@amazon.com>

src/backport.ts

@@ -154,7 +154,7 @@ const getFailedBackportCommentBody = ({
   errorMessage: string;
   head: string;
 }) => {
-  const worktreePath = `.worktrees/backport-${base}`;
+  const worktreePath = `../.worktrees/backport-${base}`;
   return [
     `The backport to \`${base}\` failed:`,
     "```",
@@ -167,15 +167,15 @@ const getFailedBackportCommentBody = ({
     "# Create a new working tree",
     `git worktree add ${worktreePath} ${base}`,
     "# Navigate to the new working tree",
-    `cd ${worktreePath}`,
+    `pushd ${worktreePath}`,
     "# Create a new branch",
     `git switch --create ${head}`,
     "# Cherry-pick the merged commit of this pull request and resolve the conflicts",
     `git cherry-pick -x --mainline 1 ${commitSha}`,
     "# Push it to GitHub",
     `git push --set-upstream origin ${head}`,
     "# Go back to the original working tree",
-    "cd ../..",
+    "popd",
     "# Delete the working tree",
     `git worktree remove ${worktreePath}`,
     "```",