bug-reference-url-format is a variable defined in `bug-reference.el'.
Its value is

  • This variable is safe as a file local variable if its value
    satisfies the predicate which is a byte-compiled expression.

Format used to turn a bug number into a URL.
The bug number is supplied as a string, so this should have a single %s.
This can also be a function designator; it is called without arguments
and should return a string.
It can use `match-string' to get parts matched against
`bug-reference-bug-regexp', specifically:
1. issue kind (bug, patch, rfe &c)
2. issue number.

There is no default setting for this, it must be set per file.
If you set it to a symbol in the file Local Variables section,
you need to add a `bug-reference-url-format' property to it:
(put 'my-bug-reference-url-format 'bug-reference-url-format t)
so that it is considered safe, see `enable-local-variables'.