abi_check: Allow checking current checkout

Without a "--detach" option, git worktree will refuse to checkout a branch
that's already checked out. This makes the abi_check.py script not very
useful for checking the currently checked out branch, as git will error
that the branch is already checked out. Add the "--detach" option to check
out the new temporary worktree in detached head mode. This is acceptable
because we aren't planning on working on the branch and just want a
checkout to do ABI checking from.
diff --git a/scripts/abi_check.py b/scripts/abi_check.py
index 2a90b68..d458d56 100755
--- a/scripts/abi_check.py
+++ b/scripts/abi_check.py
@@ -76,7 +76,7 @@
         )
         git_worktree_path = tempfile.mkdtemp()
         worktree_process = subprocess.Popen(
-            [self.git_command, "worktree", "add", git_worktree_path, git_rev],
+            [self.git_command, "worktree", "add", "--detach", git_worktree_path, git_rev],
             cwd=self.repo_path,
             stdout=subprocess.PIPE,
             stderr=subprocess.STDOUT