fix: Transaction-time check for external mods

At transaction-execute time, StGit checks to see if any intervening changes
to the branch were made with external tools (i.e. with git commands). If
the branch head has changed, StGit records an updated stack state that
reflects this new head.

The external modification check was failing to trigger when no patches were
applied even if the branch head had changed. This test is repaired.

Author: jpgrayson

src/stack/stack.rs

@@ -166,12 +166,12 @@ impl<'repo> Stack<'repo> {
 
     /// Check whether the stack's recorded head matches the branch's head.
     pub fn is_head_top(&self) -> bool {
-        self.state.applied.is_empty() || self.state.head.id() == self.branch_head.id()
+        self.state.head.id() == self.branch_head.id()
     }
 
     /// Return an error if the stack's recorded head differs from the branch's head.
     pub fn check_head_top_mismatch(&self) -> Result<()> {
-        if self.is_head_top() {
+        if self.state.applied.is_empty() || self.is_head_top() {
             Ok(())
         } else {
             Err(anyhow!(