tests/test-diff-newlines.t

doc: translate from :hg:`help config` to a valid link to hgrc.5.html

Before this patch, ":hg:`help config`" in online help text is
translated to a link to "hg.1.html#config" in HTML, even though actual
"hg help config" shows not help for "hg config" command but "config"
help topic, and all of current ":hg:`help config`" expects the latter.

This patch translates from ":hg:`help config`" in online help text to
a link to "hgrc.5.html" in HTML as expected.

This patch also allows ":hg:`help -c COMMAND`" style to link
"hg.1.html#COMMAND" for readability.

  $ hg init

  $ $PYTHON -c 'file("a", "wb").write("confuse str.splitlines\nembedded\rnewline\n")'
  $ hg ci -Ama -d '1 0'
  adding a

  $ echo clean diff >> a
  $ hg ci -mb -d '2 0'

  $ hg diff -r0 -r1
  diff -r 107ba6f817b5 -r 310ce7989cdc a
  --- a/a	Thu Jan 01 00:00:01 1970 +0000
  +++ b/a	Thu Jan 01 00:00:02 1970 +0000
  @@ -1,2 +1,3 @@
   confuse str.splitlines
   embedded\r (no-eol) (esc)
  newline
  +clean diff