fix: source syncolors.vim before startup scripts

This fixes an issue (#12573) where colorscheme files are sourced twice
upon startup. This occurs when the startup script calls `:colorscheme`,
which sets the `g:colors_name` global variable. When syntax highlighting
is enabled in `syn_maybe_enable()` the `syntax.vim` script is sourced
which in turn sources `synload.vim`. This script checks to see if
`g:colors_name` is set and, if so, runs

    exe "colors " . colors_name

This is done to ensure that highlight groups are defined before enabling
the syntax highlighting engine.

Instead, source syncolors.vim before the startup scripts which sets up
default highlights and only load the full syntax engine after
the startup scripts or when the user runs `:syntax on`. Add a guard
variable `did_syncolor` to prevent syncolor.vim from being sourced
twice and remove the line mentioned above from synload.vim so that
the colorscheme file is not re-sourced when the syntax engine is loaded.

runtime/syntax/syncolor.vim

@@ -25,6 +25,8 @@ else
   endif
 endif
 
+let did_syncolor = 1
+
 " Many terminals can only use six different colors (plus black and white).
 " Therefore the number of colors used is kept low. It doesn't look nice with
 " too many colors anyway.

runtime/syntax/synload.vim

@@ -14,10 +14,8 @@ endif
 " let others know that syntax has been switched on
 let syntax_on = 1
 
-" Set the default highlighting colors.  Use a color scheme if specified.
+" Set the default highlighting colors
-if exists("colors_name")
+if !exists("colors_name") && !exists("did_syncolor")
-  exe "colors " . colors_name
-else
   runtime! syntax/syncolor.vim
 endif
 

src/nvim/main.c

@@ -367,11 +367,19 @@ int main(int argc, char **argv)
   // Execute --cmd arguments.
   exe_pre_commands(&params);
 
+  // If using the runtime (-u is not NONE), enable syntax & filetype plugins.
+  bool enable_syntax =
+    (params.use_vimrc == NULL || !strequal(params.use_vimrc, "NONE"));
+
+  // Source syncolor.vim to set up default UI highlights
+  if (enable_syntax) {
+    source_runtime((char_u *)"syntax/syncolor.vim", DIP_ALL);
+  }
+
   // Source startup scripts.
   source_startup_scripts(&params);
 
-  // If using the runtime (-u is not NONE), enable syntax & filetype plugins.
+  if (enable_syntax) {
-  if (params.use_vimrc == NULL || !strequal(params.use_vimrc, "NONE")) {
     // Does ":filetype plugin indent on".
     filetype_maybe_enable();
     // Sources syntax/syntax.vim, which calls `:filetype on`.