Checker is blind to changes in multi-line strings

[johncarney]

Currently the checker only considers changes on key lines, meaning it will overlook changes in multi-line strings.

Possible solutions

We could modify LocaleFileKeyMap#key_at so it returns the nearest key preceding the given line, and consider all changed non-comment lines when examining the diffs. I can see some potential issues with this in how deletions are handled.

I think a more robust solution is to use git diff to identify changed files, but do all the change analysis by loading the old (ie. origin/master) and new versions of the changed files and hash-diffing them.