It matches #[0-9]+ in commit messages.
For example
 * Fix for #12
 * Code review for #56
 * Test for #15, Review on #54, Fix for #42

 It only links to valid issues (existing and belonging to the current project)
 It does not add any link to the commit in the issue page, it only consists in parsing the commit message when displayed.

 This can be considere as a primary work for the issue #155 on gitlabhq/gitlabhq.