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.
$ hg init
$ echo This is file a1 > a
$ hg add a
$ hg commit -m "commit #0"
$ ls
a
$ echo This is file b1 > b
$ hg add b
$ hg commit -m "commit #1"
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
B should disappear
$ ls
a