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/hunkUndoNearbyGitDiff.expected

@@ -0,0 +1,13 @@
+diff --git a/test/fixture.txt b/test/fixture.txt
+index f5c6aff..3fbde56 100644
+--- a/test/fixture.txt
++++ b/test/fixture.txt
+@@ -1,5 +1,8 @@
+ a
+ b
++x
++y
++z
+ c
+ d
+ e