Fix emacs indentation of multi-line match patterns

Aligns to the same column if the previous line ends in a single '|' (but
not a '||').
Emacs: stay at the correct position when indenting

When indenting a non-blank line, stay at the same cursor position
relative to the content after indenting.
Emacs: indent relative to enclosing block

This changes the indent to calculate positions relative to the enclosing
block (or braced/parenthesized expression), rather than by an absolute
nesting level within the whole file.  This allows things like this to
work:

    let x =
        match expr {
            Pattern => ...
        }

With the old method, only one level of nesting would be added within the
match braces, so "Pattern" would have ended up aligned with the match.

The other change is that multiple parens/braces on the same line only
increase the indent once.  This is a very common case for passing
closures/procs.  The absolute nesting method would do this:

    spawn(proc() {
            // Indented out two indent levels...
    })

whereas the code in this commit does this:

    spawn(proc() {
        // Indented out only one level...
    })
Fix typo in emacs ERT test
auto merge of #12486 : MicahChalmer/rust/emacs-fixes-round-3, r=brson

I've added details in the description of each comment as to what it does, which I won't redundantly repeat here in the PR.  They all relate to indentation in the emacs rust-mode.

What I will note here is that this closes #8787.  It addresses the last remaining case (not in the original issue description but in a comment), of indenting `match` statements.  With the changes here, I believe every problem described in the issue description or comments of #8787 is addressed.