Syntax highlighting error with Bibtex files

cc'ed Bernd Feige as maintainer of bib.tex syntax file. Fix can be found in
branch https://github.com/WPettersson/vim/tree/bibtex-syntax-escaped-braces

There appears to be a bug in the syntax highlighting for Bibtex files (*.bib).
Note that this bug has nothing to do with compiling anything. For any fields
that contain set-theoretic notation (like \{a,b\}) the syntax file will see the
(escaped) opening brace, but never the closing brace (as the closing brace
matches skip). For instance:

TITLE = {The set $\{a,b\}$ has 2 elements},

This results in all syntax highlighting not functioning after such an entry.
These escaped braces should only occur inside math mode (inside pairs of $'s),
so my own easy fix is to add a bibMath region as follows

syn region bibMath contained start=/\$/ end=/\$/ skip=/\(\\\$\)/

and then add bibMath to the bibVarContents cluster

syn cluster bibVarContents contains=bibUnescapedSpecial,bibMath,bibBrace,bibParen

Note that bibMath does not contain bibVarContents. Basically I just ignore
anything inside a section of math, to avoid trying to match up all the escaped
brackets, parentheses and braces inside the math.

Another possible solution would be to make sure that the start of the bibBrace
region doesn't match \{ but it feels like that should also be then extended to
bibParents and possibly further, and I don't know how well the vim syntax
engine would perform with a negative lookbehind regex. My fix seems more
appropriate in the sense that I don't think the Bibtex syntax file should try
to match all possible Latex syntax.


The above example is faked, but this does really come up. One example that I've
met with can be seen at https://ewpettersson.se/sample.bib which is the bibtex
entry for http://onlinelibrary.wiley.com/doi/10.1002/jcd.20001/abstract

For the record, I'm running vim-7.4.769 under Gentoo, but I did check that the
latest copy of bib.vim in Github also has this issue.

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