Preserve alternate buffer when processing realtime diff.

See #238.

autoload/gitgutter/diff.vim

@@ -30,7 +30,7 @@ function! gitgutter#diff#run_diff(realtime, use_external_grep)
       let buff_file .= '.'.extension
     endif
     let cmd .= 'git show '.blob_name.' > '.blob_file.' && '
-    execute 'silent write' buff_file
+    execute 'keepalt silent write' buff_file
   endif
 
   let cmd .= 'git diff --no-ext-diff --no-color -U0 '.g:gitgutter_diff_args.' -- '

test/helper.vim

@@ -20,3 +20,8 @@ function! DumpGitDiffStaged(filename)
   call system('git diff --staged fixture.txt > '.a:filename.'.actual')
 endfunction
 
+function! Dump(text, filename)
+  let msg = type(a:text) == 1 ? split(a:text, '\n') : a:text
+  call writefile(msg, a:filename.'.actual', 'a')
+endfunction
+

test/keepAlt.expected

@@ -0,0 +1,3 @@
+buffer: fixture.txt
+altbuffer: 
+altbuffer: 

test/testKeepAlt.vim

@@ -0,0 +1,12 @@
+source helper.vim
+call Setup()
+
+enew
+execute "normal! \<C-^>"
+call Dump('buffer: '.bufname(''), 'keepAlt')
+call Dump('altbuffer: '.bufname('#'), 'keepAlt')
+
+normal ggx
+doautocmd CursorHold
+call Dump('altbuffer: '.bufname('#'), 'keepAlt')
+