Auto merge of #2106 - micbou:vimdiff, r=puremourning

[READY] Do not disable YCM when Vim is started in diff mode

Since we don't really know why YCM is disabled when Vim is started in diff mode (the [relevant commit](a6d5979b08) is 4 years old) and I couldn't find any issues with YCM enabled in this mode, I propose to revert this commit.

See issue #2104 and [relevant thread on ycm-users list](https://groups.google.com/forum/?hl=en#!topic/ycm-users/cdqPJH5QrBs).

Fixes issue #2104.

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autoload/youcompleteme.vim

@@ -54,11 +54,6 @@ endfunction
 
 
 function! youcompleteme#Enable()
-  " When vim is in diff mode, don't run
-  if &diff
-    return
-  endif
-
   call s:SetUpBackwardsCompatibility()
 
   " This can be 0 if YCM libs are old or -1 if an exception occured while