Fix rollback bug in SymbolicReference.set_reference

This fixes the initialization of the "ok" flag by setting it to
False before the operation that might fail is attempted.

(This is a fix for the *specific* problem reported in gitpython-developers#1669 about
how the pattern SymbolicReference.set_reference attempts to use
with LockedFD is not correctly implemented.)

Author: EliahKagan

Diff for: git/refs/symbolic.py

@@ -370,7 +370,7 @@ def set_reference(
 
         lfd = LockedFD(fpath)
         fd = lfd.open(write=True, stream=True)
-        ok = True
+        ok = False
         try:
             fd.write(write_value.encode("utf-8") + b"\n")
             lfd.commit()