tests/test-diffdir.t

subrepo: append subrepo path to subrepo error messages

This change appends the subrepo path to subrepo errors. That is, when there
is an error performing an operation a subrepo, rather than displaying a message
such as:

pushing subrepo MYSUBREPO to PATH
searching for changes
abort: push creates new remote head HEADHASH!
hint: did you forget to merge? use push -f to force

mercurial will show:

pushing subrepo MYSUBREPO to PATH
searching for changes
abort: push creates new remote head HEADHASH! (in subrepo MYSUBREPO)
hint: did you forget to merge? use push -f to force

The rationale for this change is that the current error messages make it hard
for TortoiseHg (and similar tools) to tell the user which subrepo caused the
push failure.

The "(in subrepo MYSUBREPO)" message has been added to those subrepo methods
were it made sense (by using a decorator). We avoid appending "(in subrepo XXX)"
multiple times when subrepos are nexted by throwing a "SubrepoAbort" exception
after the extra message is appended. The decorator will then "ignore" (i.e. just
re-raise) the exception and never add the message again.

A small drawback of this method is that part of the exception trace is lost when
the exception is catched and re-raised by the annotatesubrepoerror decorator.

Also, because the state() function already printed the subrepo path when it
threw an error, that error has been changed to avoid duplicating the subrepo
path in the error message.

Note that I have also updated several subrepo related tests to reflect these
changes.

  $ hg init
  $ touch a
  $ hg add a
  $ hg ci -m "a"

  $ echo 123 > b
  $ hg add b
  $ hg diff --nodates
  diff -r 3903775176ed b
  --- /dev/null
  +++ b/b
  @@ -0,0 +1,1 @@
  +123

  $ hg diff --nodates -r tip
  diff -r 3903775176ed b
  --- /dev/null
  +++ b/b
  @@ -0,0 +1,1 @@
  +123

  $ echo foo > a
  $ hg diff --nodates
  diff -r 3903775176ed a
  --- a/a
  +++ b/a
  @@ -0,0 +1,1 @@
  +foo
  diff -r 3903775176ed b
  --- /dev/null
  +++ b/b
  @@ -0,0 +1,1 @@
  +123

  $ hg diff -r ""
  hg: parse error: empty query
  [255]
  $ hg diff -r tip -r ""
  hg: parse error: empty query
  [255]