I've tried not to replicate existing behaviour from already great plugins:
Install with your favourite vim plugin manager:
Plug 'SevereOverfl0w/deoplete-github'
Add it as a deoplete source:
let g:deoplete#sources = {}
let g:deoplete#sources.gitcommit=['github']
Deoplete also needs a little configuration:
let g:deoplete#keyword_patterns = {}
let g:deoplete#keyword_patterns.gitcommit = '.+'
call deoplete#util#set_pattern(
\ g:deoplete#omni#input_patterns,
\ 'gitcommit', [g:deoplete#keyword_patterns.gitcommit])
In your git commit message: