Improved error when push conflicts with untracked

When pushing a patch that adds a file that conflicts with an untracked file
in the working tree, the error stated "Index/worktree dirty" without
further elaboration.

The error message from `git merge-recursive` is now displayed. This message
indicates the particular untracked files that prevent the push's merge from
completing.

Also add test cases to ensure this behavior.

Addresses #193

Author: jpgrayson

src/stack/transaction.rs

@@ -1149,9 +1149,9 @@ impl<'repo> StackTransaction<'repo> {
                         push_status = PushStatus::Conflict;
                         ours
                     }
-                    Err(_) => {
+                    Err(e) => {
                         return Err(Error::TransactionHalt {
-                            msg: "Index/worktree dirty".to_string(),
+                            msg: format!("{e:#}"),
                             conflicts: false,
                         }
                         .into())

t/t1212-push-untracked.sh

@@ -0,0 +1,33 @@
+#!/bin/sh
+
+test_description='Test push conflicting with untracked files'
+
+. ./test-lib.sh
+
+test_expect_success 'Initialize the StGit repository' '
+    stg init
+'
+
+test_expect_success 'Use pop --spill' '
+    echo aaa > a.txt &&
+    echo bbb > b.txt &&
+    stg add a.txt b.txt &&
+    stg new -rm patch
+    stg pop --spill &&
+    git reset &&
+    stg add b.txt &&
+    stg new -rm add-b &&
+    conflict stg push 2>err &&
+    grep "untracked working tree files would be overwritten by merge" err &&
+    grep "a.txt" err &&
+    stg delete add-b &&
+    rm -f a.txt b.txt
+'
+
+test_expect_success 'Use pop and manually create untracked file' '
+    echo aaa > a.txt &&
+    command_error stg push 2>err &&
+    grep "Untracked working tree file .a\.txt. would be overwritten by merge" err
+'
+
+test_done