Use legitimate (existing) object IDs in tests so that we have the
ability to turn on strict object validation when running tests.

Remove most of the silly warnings

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

When formatting a patch as email we do not include the commit's
message in the formatted patch output. Implement this and add a
test that verifies behavior.

Diff: Honor `core.symlinks=false` and fake symlinks

When `core.symlinks = false`, we write the symlinks content (target)
to a regular file.  We should ensure that when we later see that
regular file, we treat it specially - and that changing that regular
file would actually change the symlink target.  (For compatibility
with Git for Windows).