mq: smarter handling of plain headers

6333412245ec and 5ccced6eab0b fixed issue4453 with a simple insertplainheader
function that fixed the regression but didn't make the implementation more
stable.

Now we introduce plain header handling similar to how we handle hg patches. The
whole header is scanned for fields to update while determining the best
position for inserting the field if it is missing. It also makes sure there is
an empty line between headers and body.

hgext/mq.py

@@ -114,6 +114,12 b' HGHEADERS = ['
     '# Node ID ',
     '# Parent  ', # can occur twice for merges - but that is not relevant for mq
     ]
+# The order of headers in plain 'mail style' patches:
+PLAINHEADERS = {
+    'from': 0,
+    'date': 1,
+    'subject': 2,
+    }
 
 def inserthgheader(lines, header, value):
     """Assuming lines contains a HG patch header, add a header line with value.
@@ -156,9 +162,40 b' def inserthgheader(lines, header, value)'
     return lines
 
 def insertplainheader(lines, header, value):
-    if lines and lines[0] and ':' not in lines[0]:
-        lines.insert(0, '')
-    lines.insert(0, '%s: %s' % (header, value))
+    """For lines containing a plain patch header, add a header line with value.
+    >>> insertplainheader([], 'Date', 'z')
+    ['Date: z']
+    >>> insertplainheader([''], 'Date', 'z')
+    ['Date: z', '']
+    >>> insertplainheader(['x'], 'Date', 'z')
+    ['Date: z', '', 'x']
+    >>> insertplainheader(['From: y', 'x'], 'Date', 'z')
+    ['From: y', 'Date: z', '', 'x']
+    >>> insertplainheader([' date : x', ' from : y', ''], 'From', 'z')
+    [' date : x', 'From: z', '']
+    >>> insertplainheader(['', 'Date: y'], 'Date', 'z')
+    ['Date: z', '', 'Date: y']
+    >>> insertplainheader(['foo: bar', 'DATE: z', 'x'], 'From', 'y')
+    ['From: y', 'foo: bar', 'DATE: z', '', 'x']
+    """
+    newprio = PLAINHEADERS[header.lower()]
+    bestpos = len(lines)
+    for i, line in enumerate(lines):
+        if ':' in line:
+            lheader = line.split(':', 1)[0].strip().lower()
+            lprio = PLAINHEADERS.get(lheader, newprio + 1)
+            if lprio == newprio:
+                lines[i] = '%s: %s' % (header, value)
+                return lines
+            if lprio > newprio and i < bestpos:
+                bestpos = i
+        else:
+            if line:
+                lines.insert(i, '')
+            if i < bestpos:
+                bestpos = i
+            break
+    lines.insert(bestpos, '%s: %s' % (header, value))
     return lines
 
 class patchheader(object):

tests/test-mq-header-date.t

@@ -412,8 +412,8 b''
   1: Three (again) - test
   0: [mq]: 1.patch - test
   ==== qref -d
+  From: jane
   Date: 12 0
-  From: jane
   
   diff -r ... 6
   --- /dev/null
@@ -465,8 +465,8 b''
   1: Three (again) - test
   0: [mq]: 1.patch - test
   ==== qref -u -d
+  From: john
   Date: 14 0
-  From: john
   
   diff -r ... 8
   --- /dev/null
@@ -495,8 +495,8 b''
   1: Three (again) - test
   0: [mq]: 1.patch - test
   ==== qref -u -d
+  From: john
   Date: 15 0
-  From: john
   
   Nine