Allow vimfiles to work with native macOS vim diffopt

CHANGES.txt

@@ -7,6 +7,12 @@ Significant changes to vimfiles (newest changes first).
 Changes marked with "BREAKING CHANGE" indicate an incompatible change that may
 require adjustment of per-user settings.
 
+2021-11-02
+==========
+
+- Fix diffopt errors when using native macOS vim, see
+  https://github.com/thoughtbot/dotfiles/issues/655
+
 2021-10-19
 ==========
 

vimrc

@@ -985,10 +985,11 @@ set updatetime=1000
 " Disallow octal numbers for increment/decrement (CTRL-a/CTRL-x).
 set nrformats-=octal
 
-" Perform vertically split diffs by default.
-set diffopt+=vertical
-
-if v:version > 801 || (v:version == 801 && has('patch-8.1.0360'))
+if has('mac') && $VIM == '/usr/share/vim'
+    " Built-in vim doesn't support diffopt internal options
+    " ref: https://github.com/thoughtbot/dotfiles/issues/655#issuecomment-605019271
+    set diffopt-=internal
+elseif v:version > 801 || (v:version == 801 && has('patch-8.1.0360'))
     " Use the new internal diff feature with options:
     " - indent-heuristic: uses indentation to improve diffs.
     " - algorithm:histogram: an improved patience algorithm as used in Git.
@@ -997,6 +998,9 @@ if v:version > 801 || (v:version == 801 && has('patch-8.1.0360'))
     set diffopt+=algorithm:histogram
 endif
 
+" Perform vertically split diffs by default.
+set diffopt+=vertical
+
 " Unfortunately, do to some redirection that Vim uses underneath the hood, it
 " can hide an error status of a command.  This helps to preserve the error
 " status so v:shell_error and the command can be tested for the success.