Without this change, compiling with gcc and pedantic generates warning:
ISO C does not allow extra ‘;’ outside of a function.

revwalk: detect when we're out of interesting commits

When walking backwards and marking parents uninteresting, make sure we
detect when the list of commits we have left has run out of
uninteresting commits so we can stop marking commits as
uninteresting. Failing to do so can mean that we walk the whole history
marking everything uninteresting, which eats up time, CPU and IO for
with useless work.

While pre-marking does look for this, we still need to check during the
main traversal as there are setups for which pre-marking does not leave
enough information in the commits. This can happen if we push a commit
and hide its parent.

When a commit is first set as unintersting and then pushed, we must take
care that we do not put it into the commit list as that makes us return
at least that commit (but maybe more) as we've inserted it into the list
because we have the assumption that we want anything in the commit list.

Keep the definitions in the headers, while putting the declarations in
the C files. Putting the function definitions in headers causes
them to be duplicated if you include two headers with them.

Introduce some helper macros to test integer overflow from arithmetic
and set error message appropriately.

There are some combination of objects and target types which we know
cannot be fulfilled. Return EINVALIDSPEC for those to signify that there
is a mismatch in the user-provided data and what the object model is
capable of satisfying.

If we start at a tag and in the course of peeling find out that we
cannot reach a particular type, we return EPEEL.

This should have been included when implementing the feature but was
missed.