Allow changes to be reverse applied from previous commit by hunk/line/file

SourceTree already supports per-hunk and per-line staging/unstaging, and also supports 'Amend previous commit'. However one feature that GitX has is that when you click the 'Amend' checkbox, it displays all the previous changes in the 'Staged' box and allows you to unstage them (i.e. reverse apply them). This is pretty handy for cleaning up commits before pushing.

https://plus.google.com/u/0/110277469483406332394/posts/4m2tNzLmXHr

Without necessarily doing this exactly the same way, it would be useful to expose this functionality. A better way in fact would be to allow reverse-apply on any set of changes in the log, by hunk and by line (or by file). This could be used to undo changes more specifically than backing out an entire commit, can apply to both git and hg, and isn't limited to simply the previous commit.