mq: when setting message in plain mode, separate it from header (issue4453)

Fix inconsistent handling of plain header separation in mq patcheader - and
contrary to c87f2a5a6e49, do it in the direction of having an empty line
between header and description. Plain patches are like mails and should thus
have an empty line between headers and body in compliance with RFC 822 3.1.

hgext/mq.py

@@ -293,7 +293,10 b' class patchheader(object):'
         if self.comments:
             self._delmsg()
         self.message = [message]
-        self.comments += self.message
+        if message:
+            if self.plainmode and self.comments and self.comments[-1]:
+                self.comments.append('')
+            self.comments.append(message)
 
     def updateheader(self, prefixes, new):
         '''Update all references to a field in the patch header.

tests/test-mq-header-date.t

@@ -243,6 +243,7 b''
   now at: 1.patch
   ==== qnew -d -m
   Date: 6 0
+  
   Three
   
   1: Three - test - 6.00
@@ -250,6 +251,7 b''
   ==== qref
   adding 3
   Date: 6 0
+  
   Three
   
   diff -r ... 3
@@ -261,6 +263,7 b''
   0: [mq]: 1.patch - test - 4.00
   ==== qref -m
   Date: 6 0
+  
   Drei
   
   diff -r ... 3
@@ -272,6 +275,7 b''
   0: [mq]: 1.patch - test - 4.00
   ==== qref -d
   Date: 7 0
+  
   Drei
   
   diff -r ... 3
@@ -283,6 +287,7 b''
   0: [mq]: 1.patch - test - 4.00
   ==== qref -d -m
   Date: 8 0
+  
   Three (again)
   
   diff -r ... 3

tests/test-mq-header-from.t

@@ -176,6 +176,7 b''
   0: [mq]: 1.patch - mary
   ==== qnew -U -m
   From: test
+  
   Three
   
   2: Three - test
@@ -184,6 +185,7 b''
   ==== qref
   adding 3
   From: test
+  
   Three
   
   diff -r ... 3
@@ -196,6 +198,7 b''
   0: [mq]: 1.patch - mary
   ==== qref -m
   From: test
+  
   Drei
   
   diff -r ... 3
@@ -208,6 +211,7 b''
   0: [mq]: 1.patch - mary
   ==== qref -u
   From: mary
+  
   Drei
   
   diff -r ... 3
@@ -220,6 +224,7 b''
   0: [mq]: 1.patch - mary
   ==== qref -u -m
   From: maria
+  
   Three (again)
   
   diff -r ... 3