It seems like MingW64's size_t is defined differently than in Linux.

GCC warns by default when implicitly converting integers to pointers or
the other way round, and commit fa48d2ea7 (vector: do not malloc
0-length vectors on dup, 2018-09-26) introduced such an implicit
conversion into our vector tests. While this is totally fine in this
test, as the pointer's value is never being used in the first place, we
can trivially avoid the warning by instead just inserting a pointer for
a variable allocated on the stack into the vector.

C++ style comment ("//") are not specified by the ISO C90 standard and
thus do not conform to it. While libgit2 aims to conform to C90, we did
not enforce it until now, which is why quite a lot of these
non-conforming comments have snuck into our codebase. Do a tree-wide
conversion of all C++ style comments to the supported C style comments
to allow us enforcing strict C90 compliance in a later commit.

This is a convenience function to reverse the contents of a vector and a pqueue
in-place.

The pqueue function is useful in the case where we're treating it as a
LIFO queue.

This is just laying some groundwork for internal index changes
that I'm working on.