Find fast bug comparison method

Compare the speed of the two bug comparison methods `equal_contents` and `make_bugs_comparable`. The faster method should then be used in `merge_bugs`.

Result: `equal_contents` is slightly faster.
 * Ratio ~ 0.7 for same number of equal and unequal bugs.
 * Ratio ~ 0.3 (much faster) for only unequal bugs.
 * Ratio ~ 0.95 for only equal bugs.

{{{
#!python
BUG_CONTENTS_FIELDS = list(set(BugData._fields).difference(
                                    ["ids", "time_created", "time_modified"]))
def equal_contents(bug1, bug2):
    """
    Test if bugs are equal, but ignore fields that the repositories
    always change. (ids, time_created, time_modified)
    """
    for fname in BUG_CONTENTS_FIELDS:
        if getattr(bug1, fname) != getattr(bug2, fname):
            return False
    return True

def make_bugs_comparable(bug):
    """
    Set bugs to neutral state, so that it can be compared.
    TODO: copying two bugs for every comparison is a waste of resources
    """
    return bug._replace(ids={}, time_created=None, time_modified=None)
}}}