Bitbucket already supports marking issues on commit using the Issues service.
The issues service scans commit messages for commands which will automatically change the state of the relevant issue on the tracker.
[snip]
Examples:
“… fixes #4711 …” # marks issue as resolved
“… reopening bug 4711…” # marks issue as open
“… refs ticket 4711…” # links changeset to issue as comment
“… refs bug #4711 and #4712…” # links to multiple issues
You may need to activate it, to do so:
Services can be activated on a per-repository basis by the administrator. Go to your repository page, select the Admin tab, click Services, then select a service from the list, and add it.