Support "source-read" being emitted for included docs

Since Sphinx 7.2, the "source-read" event is emitted for included
documents (.. include directive). A document can be included multiple
times, and thus the hook cannot assume that it's called only once per
docname.

An issue in Sphinx itself affects the extension when using included
documents. Add a warning in the README.

README.rst

@@ -79,6 +79,10 @@ Caveats
 
       See also `issue #1`_.
 
+    * A regression in Sphinx version 7.2 prevents the extension from working
+      properly when using the `.. include` directive. See `Sphinx issue
+      #11643`_.
+
     * In Sphinx versions 5.0 and 5.1, there has been
       a regression in how dependencies are determined.
       This could lead to spurious dependencies
@@ -117,3 +121,4 @@ Similar stuff
     guides/feature-flags.html
 .. _issue #1: https://github.com/mgeier/sphinx-last-updated-by-git/issues/1
 .. _issue #40: https://github.com/mgeier/sphinx-last-updated-by-git/issues/40
+.. _Sphinx issue #11643: https://github.com/sphinx-doc/sphinx/issues/11643

src/sphinx_last_updated_by_git.py

@@ -276,8 +276,10 @@ def _builder_inited(app):
 
 def _source_read(app, docname, source):
     env = app.env
-    assert docname not in env.git_last_updated
-    env.git_last_updated[docname] = None
+    # Since Sphinx 7.2, the source-read hook can be called multiple times
+    # when using .. include
+    if docname not in env.git_last_updated:
+        env.git_last_updated[docname] = None
 
 
 def _env_merge_info(app, env, docnames, other):