patch: rename "header" variable into "hdr" in diff()

The "header" variable was hiding the eponymous class, hence preventing its
usage.

mercurial/patch.py

@@ -2316,13 +2316,13 b' def diff(repo, node1=None, node2=None, m'
 
     copy, if not empty, should contain mappings {dst@y: src@x} of copy
     information.'''
-    for header, hunks in diffhunks(repo, node1=node1, node2=node2, match=match,
+    for hdr, hunks in diffhunks(repo, node1=node1, node2=node2, match=match,
                                    changes=changes, opts=opts,
                                    losedatafn=losedatafn, prefix=prefix,
                                    relroot=relroot, copy=copy):
         text = ''.join(sum((list(hlines) for hrange, hlines in hunks), []))
-        if header and (text or len(header) > 1):
-            yield '\n'.join(header) + '\n'
+        if hdr and (text or len(hdr) > 1):
+            yield '\n'.join(hdr) + '\n'
         if text:
             yield text