Vim: re-enable identifier completion

Turns out that *only* semantic completion is triggered on Ctrl-Space.
Since this is slightly more annoying than the automatic popup (and
there's not a way to have manual identifier completion trigger along
with semantic completion, see ycm-core/YouCompleteMe#1114), let's use ID
completion.

.gitmodules

@@ -99,3 +99,6 @@
 [submodule "git-remote-hg"]
 	path = git-remote-hg
 	url = https://github.com/fingolfin/git-remote-hg.git
+[submodule "vim/bundle/vimomni.vim"]
+	path = vim/bundle/vimomni.vim
+	url = https://github.com/c9s/vimomni.vim.git

vimrc

@@ -200,8 +200,8 @@ if executable('fortune')
 endif
 
 " Wrestle YouCompleteMe into behaving
-" Effectively disable the identifier popup, and disable some highlighting
-let g:ycm_min_num_of_chars_for_completion = 99
+" Tweak the identifier popup, and disable some highlighting
+let g:ycm_min_num_of_chars_for_completion = 4
 let g:ycm_enable_diagnostic_highlighting = 0
 let g:ycm_autoclose_preview_window_after_insertion = 1
 let g:ycm_autoclose_preview_window_after_completion = 1