Strip whitespace in latexsub :exit-function

[non-Jedi]

According to the docstring for completion-extra-properties, the string passed to the :exit-function should be the bare text to which the field was completed, but helm-mode adds an extra space at the end of string. Since none of our latexsubs include whitespace, we can safely strip whitespace in the :exit-function and work around this helm bug.

I will file an upstring bug report against helm, but I think it's worth fixing quickly here since it's only a simple 3-line change.

Closes #204.

@tpapp, @giordano, or @FelipeLema could one of you review please? This fixes an extremely annoying bug for anyone using helm. @giordano, could you please double-check that this fixes the issue for you if you get a chance?

[non-Jedi]

For anyone curious on history, this is the same bug as emacs-helm/helm#2360. I'm not at all sure how I had helm working before given that Thierry only fixed that issue for insertion but not for what :exit-function sees. Based on what I see there, an upstream solution to this bug seems unlikely, and this workaround is relatively painless.

[tpapp]

LGTM, thanks!

[non-Jedi]

Thanks @tpapp and @FelipeLema! Merged as 2dfc869.

[giordano]

Sorry, I had a chance to test this only now! It mostly works in the sense that \alph<TAB> is replaced with α straight away (yay!), but \alpha<TAB> does nothing, \alpha remains there. Thanks for looking into this!

[non-Jedi]

Looks like helm-mode isn't calling the :exit-function when the symbol is already complete. I don't immediately see a way of working around this without an upstream fix. Will open an issue against helm once I've created a minimal reproducer.