Windows defines `timeval` with `long`, which we cannot
sanely cope with.  Instead, use a custom timeval struct.

Fix the file-mode test to expect system umask being applied to the
created file as well (it is currently applied to the directory only).
This fixes the test on systems where umask != 022.

Signed-off-by: Michał Górny <mgorny@gentoo.org>

When there's no matching index entry (for whatever reason), don't
try to dereference the null return value to get at the id.

Otherwise when we break something in the index API, the checkout
test crashes for confusing reasons and causes us to step through
it in a debugger thinking that we had broken much more than we
actually did.

Untangle git_futils_mkdir from git_futils_mkdir_ext - the latter
assumes that we own everything beneath the base, as if it were
being called with a base of the repository or working directory,
and is tailored towards checkout and ensuring that there is no
bogosity beneath the base that must be cleaned up.

This is (at best) slow and (at worst) unsafe in the larger context
of a filesystem where we do not own things and cannot do things like
unlink symlinks that are in our way.

When a file exists on disk and we're checking out a file that differs
in executableness, remove the old file.  This allows us to recreate the
new file with p_open, which will take the new mode into account and
handle setting the umask properly.

Remove any notion of chmod'ing existing files, since it is now handled
by the aforementioned removal and was incorrect, as it did not take
umask into account.