transaction: clarify the "quick abort" scenario

Right now, the transaction has a code-pass to do a "quick abort" that skip most¹
(too much) of the logic when the right condition are detected²

We are about to improve this logic in multiple aspect. We clarify the code
first.

The conditional return in `_can_quick_abort` looks a bit weird because we are
about to make them more complex very soon.

[1] actually too much
[2] actually not often enough

mercurial/transaction.py

@@ -668,16 +668,38 b' class transaction(util.transactional):'
         self._file.close()
         self._backupsfile.close()
 
+        quick = self._can_quick_abort(entries)
         try:
-            if not entries and not self._backupentries:
+            if quick:
+                self._do_quick_abort(entries)
+            else:
+                self._do_full_abort(entries)
+        finally:
+            self._journal = None
+            self._releasefn(self, False)  # notify failure of transaction
+            self._releasefn = None  # Help prevent cycles.
+
+    def _can_quick_abort(self, entries):
+        """False if any semantic content have been written on disk
+
+        True if nothing, except temporary files has been writen on disk."""
+        if entries:
+            return False
+        if self._backupentries:
+            return False
+        return True
+
+    def _do_quick_abort(self, entries):
+        """(Silently) do a quick cleanup (see _can_quick_abort)"""
+        assert self._can_quick_abort(entries)
                 if self._backupjournal:
                     self._opener.unlink(self._backupjournal)
                 if self._journal:
                     self._opener.unlink(self._journal)
-                return
 
+    def _do_full_abort(self, entries):
+        """(Noisily) rollback all the change introduced by the transaction"""
             self._report(_(b"transaction abort!\n"))
-
             try:
                 for cat in sorted(self._abortcallback):
                     self._abortcallback[cat](self)
@@ -699,10 +721,6 b' class transaction(util.transactional):'
                 self._report(
                     _(b"(failure reason: %s)\n") % stringutil.forcebytestr(exc)
                 )
-        finally:
-            self._journal = None
-            self._releasefn(self, False)  # notify failure of transaction
-            self._releasefn = None  # Help prevent cycles.
 
 
 BAD_VERSION_MSG = _(