clean: show an error message when the path is too long

clean: show an error message when the path is too long

When `lstat()` failed, `git clean` would abort without an error
message, leaving the user quite puzzled.

In particular on Windows, where the default maximum path length is
quite small (yet there are ways to circumvent that limit in many
cases), it is very important that users be given an indication why
their command failed because of too long paths when it did.

This test case makes sure that a warning is issued that would have
helped the user who reported this issue:

	https://github.com/git-for-windows/git/issues/521

Note that we temporarily set `core.longpaths = false` in the regression
test; this ensures forward-compatibility with the `core.longpaths`
feature that has not yet been upstreamed from Git for Windows.

Helped-by: René Scharfe <l.s.r@web.de>
Helped-by: SZEDER Gábor <szeder.dev@gmail.com>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>