Use "undo" instead of "revert" for discarding a hunk.

"Undo" is a better name than "revert" because:

- "revert" sounds like it has something to do with `git-revert` but they
  are entirely different;
- "undo" is consistent with vim's "undo": discarding changes and going
  back to the original.

Maintain backwards compatibility and add deprecation warnings.

Closes #306.

test/testHunkUndoNearbyHunk.vim

@@ -0,0 +1,9 @@
+source helper.vim
+call Setup()
+
+execute "normal! 2Gox\<CR>y\<CR>z"
+normal 2jdd
+normal k
+execute 'GitGutterUndoHunk'
+call DumpSigns('hunkUndoNearbySigns')
+call DumpGitDiff('hunkUndoNearbyGitDiff')