typo fix (arrachments->attachments)
Fix "undefined" filename when commenting on a changeset
Fix line numbering display issues when commenting on a line of a changeset

To associate a comment with a specific line of the changeset, we use the row number in the diff table displayed by Trac.  However, this didn't work for diffs with multiple files (if the first file had five lines, the second file's first line would be line #6) or deleted lines.

I added a displayLine variable that controls what we show to the user: the actual line number of the file, or its former line number if the line was deleted.  (We still use the absolute row number to store the comment, since otherwise we couldn't disambiguate which line the comment belongs to.)
Fix "Path" display in Code Comments table for changeset comments

Be more specific about when a comment is on a changeset vs. a file.  Also, remove the line number from the changeset for now, because it's confusing (it refers to the absolute line number of the changeset, NOT the line within the file).  To show that a comment refers to a particular line in a changeset, we add "(in filename)".
Get rid of the "is" option in attachment_info
Initialize comment path to empty

This fixes the "Comment instance has no attribute 'path'" error that prevented comments from being added to changesets, or in reply to line comments on changesets.