Add test for hunk staging.

test/helper.vim

@@ -2,6 +2,7 @@ set runtimepath+=../
 source ../plugin/gitgutter.vim
 
 function! Setup()
+  call system('git reset HEAD fixture.txt')
   call system('git checkout fixture.txt')
   edit! fixture.txt
   sign unplace *
@@ -13,3 +14,6 @@ function! DumpSigns(filename)
   redir END
 endfunction
 
+function! DumpGitDiff(filename)
+  call system('git diff --staged fixture.txt > '.a:filename.'.out')
+endfunction

test/hunkStageGitDiff.ok

@@ -0,0 +1,13 @@
+diff --git a/test/fixture.txt b/test/fixture.txt
+index f5c6aff..ae8e546 100644
+--- a/test/fixture.txt
++++ b/test/fixture.txt
+@@ -2,7 +2,7 @@ a
+ b
+ c
+ d
+-e
++*e
+ f
+ g
+ h

test/hunkStageSigns.ok

@@ -0,0 +1,2 @@
+
+--- Signs ---

test/testHunkStage.vim

@@ -0,0 +1,7 @@
+source helper.vim
+call Setup()
+
+normal 5Gi*
+execute 'GitGutterStageHunk'
+call DumpSigns('hunkStageSigns')
+call DumpGitDiff('hunkStageGitDiff')