Add ability to search working tree

It would be very handy to be able to quickly search the working tree and/or filter the changes list like you can do already with filenames. For example if you have an issue with a method you could search for any changes involved in that method. Or if you want to tease out a commit of just its related changes.

There are a few variations of this which would all be helpful, including:
Search all the files in the working set
Search only the changes/diff in the working set (similar to git diff-index --cached -S<SEARCH STRING> HEAD)

Originally discussed here - https://answers.atlassian.com/questions/129612/is-there-a-way-to-search-the-working-tree-changes