Update git-source citation in Reference.set_object

Reference.set_object explains its handling of oldbinsha by quoting
a comment from refs.c in the Git source code. However, that comment
now appears in refs/files-backend.c in that codebase. This updates
the reference so readers can look it up and find the comment in its
surrounding context.

The commit to the git project's source code that moved the code
that includes that comment is:

git/git@7bd9bcf

Author: EliahKagan

git/refs/reference.py

@@ -86,18 +86,19 @@ def set_object(
         super(Reference, self).set_object(object, logmsg)
 
         if oldbinsha is not None:
-            # /* from refs.c in git-source
-            # * Special hack: If a branch is updated directly and HEAD
-            # * points to it (may happen on the remote side of a push
-            # * for example) then logically the HEAD reflog should be
-            # * updated too.
-            # * A generic solution implies reverse symref information,
-            # * but finding all symrefs pointing to the given branch
-            # * would be rather costly for this rare event (the direct
-            # * update of a branch) to be worth it.  So let's cheat and
-            # * check with HEAD only which should cover 99% of all usage
-            # * scenarios (even 100% of the default ones).
-            # */
+            # From refs/files-backend.c in git-source:
+            # /*
+            #  * Special hack: If a branch is updated directly and HEAD
+            #  * points to it (may happen on the remote side of a push
+            #  * for example) then logically the HEAD reflog should be
+            #  * updated too.
+            #  * A generic solution implies reverse symref information,
+            #  * but finding all symrefs pointing to the given branch
+            #  * would be rather costly for this rare event (the direct
+            #  * update of a branch) to be worth it.  So let's cheat and
+            #  * check with HEAD only which should cover 99% of all usage
+            #  * scenarios (even 100% of the default ones).
+            #  */
             self.repo.head.log_append(oldbinsha, logmsg)
         # END check if the head