[opti] Update sections in-place

This avoids unecessary deepcopies. This is a preparatory step to
reimplementing the extends algorithm. It may be that this breaks
the extends algorithm as it is currently implemented.

src/zc/buildout/buildout.py

@@ -1533,7 +1533,7 @@ class Options(DictMixin):
 
             result = _annotate_section(result, "")
             data = _annotate_section(copy.deepcopy(data), "")
-            result = _update_section(result, data)
+            _update_section(result, data)
             result = _unannotate_section(result)
             result.pop('<', None)
             return result
@@ -1972,14 +1972,13 @@ def _dists_sig(dists):
             result.append(os.path.basename(location))
     return result
 
-def _update_section(in1, s2):
-    s1 = copy.deepcopy(in1)
-    # Base section 2 on section 1; section 1 is copied, with key-value pairs
-    # in section 2 overriding those in section 1.
+def _update_section(s1, s2):
+    # Base section 2 on section 1; key-value pairs in s2 override those in s1.
     # If there are += or -= operators in s2, process these to add or subtract
     # items (delimited by newlines) from the preexisting values.
     # Sort on key, then on + and - operators, so that KEY < KEY + < KEY -, to
     # process them in this order if several are defined in the same section.
+    # Section s1 is modified in place.
     keysort = lambda x: (x[0].rstrip(' +'), x[0].endswith('+'))
     for k, v in sorted(s2.items(), key=keysort):
         if k.endswith('+'):
@@ -2000,13 +1999,12 @@ def _update_section(in1, s2):
                 s1[k] = v
     return s1
 
-def _update(in1, d2):
-    d1 = copy.deepcopy(in1)
+def _update(d1, d2):
     for section in d2:
         if section in d1:
-            d1[section] = _update_section(d1[section], d2[section])
+            _update_section(d1[section], d2[section])
         else:
-            d1[section] = copy.deepcopy(d2[section])
+            d1[section] = d2[section]
     return d1
 
 def _recipe(options):