vim-patch:7.4.503 #2178

Problem:    Cannot append a list of lines to a file.
Solution:   Add the append option to writefile(). (Yasuhiro Matsumoto)

https://code.google.com/p/vim/source/detail?r=v7-4-503

-Ported old legacy test over to
    test/functional/legacy/writefile_spec.lua
-Tests for mapping and signs from the original patch were removed since
    they have nothing to do this with feature

Tested with: make oldtest, make test on OS X.

Signed-off-by: Perry Hung <iperry@gmail.com>

src/nvim/version.c

@@ -337,7 +337,7 @@ static int included_patches[] = {
   //506 NA
   //505 NA
   //504 NA
-  //503,
+  503,
   //502,
   //501 NA
   500,