Avoid multiple syntax enable during Vim startup

[itspriddle]

If one runs vim +PlugUpdate there are two calls to syntax enable; one from plug#end() in your vimrc and another from the PlugUpdate command itself. It seems that when you use +PlugUpdate via the CLI that both calls to plug#end() happen while has('vim_starting') is true. This causes a minor issue where syntax highlighting (for the plug.vim itself) is lost when PlugUpdate finishes running.

There is a gif I found in an unrelated issue that shows this in action.

With this patch syntax highlighting is not lost when PlugUpdate finishes.

[junegunn]

Looks good, thanks. Can you update the failing assertion as well? You can run the whole test with test/run script.

[itspriddle]

Yep, should be good. Let me know if not, I havent used vader before 😄

[junegunn]

Thanks!