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The general practise is to open a bug when some task needs to be done, even if there is no "bug" involved. The idea is to provide a "thread" of discussion and a place to track development. Also does Bugzilla offer a more persistent way of storing things, ie. stuff like patches and votes are less likely to get lost on the mailinglist.
Committing Fixes For Bugs
When you commit a fix for some bug, the number of the bug should be stated in the commit message. JustinMason said once:
BTW, a tip on referring to bugs – the convention is
bug NNNN: blah blah
simply because Bugzilla has the smarts to automatically turn "bug NNNN"
into a hyperlink – and it's easily greppable.