mq: when adding headers in plain mode, separate them from message (issue4453)

c87f2a5a6e49 did a clean-up in one direction ... but we want it in the other
direction.

hgext/mq.py

@@ -151,6 +151,8 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))
     return lines
 

tests/test-mq-header-date.t

@@ -311,6 +311,7 b''
   0: [mq]: 1.patch - test
   ==== qref -d
   Date: 9 0
+  
   Four
   
   diff -r ... 4
@@ -494,6 +495,7 b''
   ==== qref -u -d
   Date: 15 0
   From: john
+  
   Nine
   
   diff -r ... 9

tests/test-mq-header-from.t

@@ -250,6 +250,7 b''
   0: [mq]: 1.patch - mary
   ==== qref -u
   From: jane
+  
   Four
   
   diff -r ... 4of