Add omni completion

[at15]

change complete function for COMMIT_EDITMSG file

• [x] show some hit when use C-X + C-O
• [x] show real hint for issues use C-X + C-O
• [x] use issue number to match
• [ ] use issue title to match

[at15]

there is a useful sample http://vimdoc.sourceforge.net/htmldoc/insert.html#complete-functions