revset: factor out getinteger() helper We have 4 revset functions that take integer arguments, and they handle their arguments in slightly different ways. This patch unifies them: - getstring() in place of getsymbol(), which is more consistent with the handling of integer revisions (both 1 and '1' are valid) - say "expects" instead of "requires" for type errors We don't need to catch TypeError since getstring() must return a string.