mq: fix update of headers that occur in the "wrong" order

Mq tried to insert headers in the right order. Sometimes it would stop
searching before checking all headers and it could thus duplicate a header
instead of replacing it.

hgext/mq.py

@@ -113,7 +113,6 b' HGHEADERS = ['
     '# Branch ',
     '# Node ID ',
     '# Parent  ', # can occur twice for merges - but that is not relevant for mq
-    '', # all lines after headers 'has' this prefix - simplifies the algorithm
     ]
 
 def inserthgheader(lines, header, value):
@@ -127,6 +126,9 b' def inserthgheader(lines, header, value)'
     ['# HG changeset patch', '# Date z', '']
     >>> inserthgheader(['# HG changeset patch', '# User y'], '# Date ', 'z')
     ['# HG changeset patch', '# User y', '# Date z']
+    >>> inserthgheader(['# HG changeset patch', '# Date x', '# User y'],
+    ...                '# User ', 'z')
+    ['# HG changeset patch', '# Date x', '# User z']
     >>> inserthgheader(['# HG changeset patch', '# Date y'], '# Date ', 'z')
     ['# HG changeset patch', '# Date z']
     >>> inserthgheader(['# HG changeset patch', '', '# Date y'], '# Date ', 'z')
@@ -136,18 +138,21 b' def inserthgheader(lines, header, value)'
     """
     start = lines.index('# HG changeset patch') + 1
     newindex = HGHEADERS.index(header)
+    bestpos = len(lines)
     for i in range(start, len(lines)):
         line = lines[i]
+        if not line.startswith('# '):
+            bestpos = min(bestpos, i)
+            break
         for lineindex, h in enumerate(HGHEADERS):
             if line.startswith(h):
-                if lineindex < newindex:
-                    break # next line
                 if lineindex == newindex:
                     lines[i] = header + value
-                else:
-                    lines.insert(i, header + value)
-                return lines
-    lines.append(header + value)
+                    return lines
+                if lineindex > newindex:
+                    bestpos = min(bestpos, i)
+                break # next line
+    lines.insert(bestpos, header + value)
     return lines
 
 def insertplainheader(lines, header, value):