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.
#require execbit
$ hg init repo
$ cd repo
$ echo foo > foo
$ chmod 644 foo
$ hg ci -qAm '644'
$ chmod 755 foo
$ hg ci -qAm '755'
reverting to rev 0
$ hg revert -a -r 0
reverting foo
$ hg st
M foo
$ hg diff --git
diff --git a/foo b/foo
old mode 100755
new mode 100644
$ cd ..