bug-reference-bug-regexp now defines a contract for the overlay region

bug-reference-bug-regexp now defines a contract for the overlay region

Formerly, bug-reference-fontify placed the overlay on the complete
match of bug-reference-bug-regexp.  That made it impossible to encode
constraints like "must not match at BOL" in the regexp without messing
up fontification.  Therefore, now it establishes the contract that
subexpression 1 defines the overlay region.  Subexpression 2 must
still match the part of the bug reference injected into
bug-reference-url-format if that's a string.  If its a function, the
interpretation of subexpressions > 1 is up to the function.

For backwards compatibility, bug-reference-fontify checks if the
bounds of subexpression 2..10 are within the bounds of subexpession
1.  If not, or subexpression 1 doesn't even exist/match, we fall back
to placing the overlay from (match-beginning 0) to (match-end 0) but
issue a warning.

* lisp/progmodes/bug-reference.el (bug-reference-bug-regexp): Document
contract that subexpression 1 defines the overlay region and adapt the
default value accordingly.
(bug-reference--nonconforming-regexps): New internal variable.
(bug-reference--overlay-bounds): New function.
(bug-reference-fontify): Place overlay on subexpression 1's bounds if
bug-reference-bug-regexp conforms to the documented contract.
(bug-reference--setup-from-vc-alist): Adapt regexps to new contract.
* doc/emacs/maintaining.texi (Bug Reference): Adapt regexp used in
example.