Fix for invalid matching symbol in vim keymap.

When the matching symbol of a closing bracket is on a different
line, it will match to the incorrect opening bracket. For example,
go to http://codemirror.net/demo/vim.html and place the cursor on
the closing brace at line 11 and press '%'.  It will incorrectly
match the opening brace on line 4 instead of the brace on line 8.
This patch fixes that by starting the line search at the *end* of
the line when searching backwards.