[READY] Improve diagnostic matches display
There are two issues with how we display diagnostic matches. The first issue is that if the current buffer contains diagnostic matches and is split in a new window, all the matches are cleared and only matches in the new window are shown. The second issue is that if a new buffer with no diagnostic support is open in the current window and that window already contained matches then the matches are still displayed. Here's an illustration of both issues (signs are disabled):
The solution is to add an autocommand that updates matches on the `BufEnter` and `WinEnter` events. Here's the result:
Note that it's not perfect as multiple windows of the same buffer won't be updated simultaneously. Supporting that scenario is rather tricky because we would need to go through all the windows to update the matches and switching windows can lead to a lot of issues (like interrupting visual mode) so we don't.
This PR also improves how we update matches by only displaying matches that are not already present and then clearing the remaining ones (similarly to what we do with signs; see PR https://github.com/Valloric/YouCompleteMe/pull/2915) instead of always clearing all the matches then displaying the new ones.
<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/valloric/youcompleteme/2922)
<!-- Reviewable:end -->
Evaluating the values of g:ycm_extra_conf_vim_data may raise a Python exception
(e.g. one of the values is not defined). Since that option is parsed each time
a request is sent, such exception makes the editor almost unusable as each key
press is printing a Python traceback to the user. Catch and log the exception.
Reduce the time spent to build the request when there are a lot of buffers by:
- using the options property on the buffer object to get the mod variable
instead of evaluating getbufvar;
- not computing the buffer filepath if the buffer is not modified;
- passing the number of the unloaded buffer instead of its filepath on the
BufferUnload event. Getting the Python buffer object from its number is
easier than from its filepath.
The 'hidden' option is a global option, not a buffer one. If this option is
false, we should check if the 'bufhidden' option, which is local to the buffer,
is set to 'hide'. If so, the buffer can be hidden despite the 'hidden' option
being false.
Bring fully asynchronous completion by polling for completions with a timer
then calling completefunc once the completions are ready. Use the start column
returned by the server in completefunc. Immediately display the last completion
on the TextChangedI event to prevent the popup menu disappearing while waiting
for the completions. Handle the TextChangedI event not being triggered while
the completion menu is open by closing the menu when inserting a character
through the InsertCharPre event, and when deleting a character on the <BS> and
<C-h> keys.
