mingw: special-case index entries for symlinks with buggy size

mingw: special-case index entries for symlinks with buggy size

In https://github.com/git-for-windows/git/pull/2637, we fixed a bug
where symbolic links' target path sizes were recorded incorrectly in the
index. The downside of this fix was that every user with tracked
symbolic links in their checkouts would see them as modified in `git
status`, but not in `git diff`, and only a `git add <path>` (or `git add
-u`) would "fix" this.

Let's do better than that: we can detect that situation and simply
pretend that a symbolic link with a known bad size (or a size that just
happens to be that bad size, a _very_ unlikely scenario because it would
overflow our buffers due to the trailing NUL byte) means that it needs
to be re-checked as if we had just checked it out.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>