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

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

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).

Ensure that a diff with the workdir is not erroneously returning
directories.

Some nicer refactoring for index iteration walks.

The index iterator doesn't binary search through the pathlist space,
since it lacks directory entries, and would have to binary search
each index entry and all its parents (eg, when presented with an index
entry of `foo/bar/file.c`, you would have to look in the pathlist for
`foo/bar/file.c`, `foo/bar` and `foo`).  Since the index entries and the
pathlist are both nicely sorted, we walk the index entries in lockstep
with the pathlist like we do for other iteration/diff/merge walks.

When using literal pathspecs in diff with `GIT_DIFF_DISABLE_PATHSPEC_MATCH`
turn on the faster iterator pathlist handling.

Updates iterator pathspecs to include directory prefixes (eg, `foo/`)
for compatibility with `GIT_DIFF_DISABLE_PATHSPEC_MATCH`.

Document that `GIT_DIFF_PATHSPEC_DISABLE` is not necessarily about
explicit path matching, but also includes matching of directory
names.  Enforce this in a test.

Test that workdir diffs, when presented with UPDATE_INDEX, only
write the index when they actually make a change.