Add test for revert subsequent nearby hunk.

test/hunkRevertNearbyGitDiff.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

test/hunkRevertNearbySigns.expected

@@ -0,0 +1,6 @@
+
+--- Signs ---
+Signs for fixture.txt:
+    line=3  id=3000  name=GitGutterLineAdded
+    line=4  id=3001  name=GitGutterLineAdded
+    line=5  id=3002  name=GitGutterLineAdded

test/testHunkRevertNearbyHunk.vim

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