t/Makefile: add a rule to re-run previously-failed tests

This patch automates the process of determining which tests failed
previously and re-running them.

While developing patch series, it is a good practice to run the test
suite from time to time, just to make sure that obvious bugs are caught
early.  With complex patch series, it is common to run `make -j15 -k
test`, i.e.  run the tests in parallel and *not* stop at the first
failing test but continue. This has the advantage of identifying
possibly multiple problems in one big test run.

It is particularly important to reduce the turn-around time thusly on
Windows, where the test suite spends 45 minutes on the computer on which
this patch was developed.

It is the most convenient way to determine which tests failed after
running the entire test suite, in parallel, to look for left-over "trash
directory.t*" subdirectories in the t/ subdirectory. However, those
directories might live outside t/ when overridden using the
--root=<directory> option, to which the Makefile has no access. The next
best method is to grep explicitly for failed tests in the test-results/
directory, which the Makefile *can* access.

Please note that the often-recommended `prove` tool requires Perl, and
that opens a whole new can of worms on Windows. As no native Windows Perl
comes with Subversion bindings, we have to use a Perl in Git for Windows
that uses the POSIX emulation layer named MSYS2 (which is a portable
version of Cygwin). When using this emulation layer under stress, e.g.
when running massively-parallel tests, unexplicable crashes occur quite
frequently, and instead of having a solution to the original problem, the
developer now has an additional, quite huge problem. For that reason, this
developer rejected `prove` as a solution and went with this patch instead.

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

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