Windows: a test_cmp that is agnostic to random LF <> CRLF conversions

Windows: a test_cmp that is agnostic to random LF <> CRLF conversions

In a number of tests, output that was produced by a shell script is
compared to expected output using test_cmp. Unfortunately, the MSYS bash--
when invoked via git, such as in hooks--converts LF to CRLF on output
(as produced by echo and printf), which leads to many false positives.

Implements a diff tool that undoes the converted CRLF. To avoid that
sub-processes are spawned (which is very slow on Windows), the tool is
implemented as a shell function. Diff is invoked as usual only when a
difference is detected by the shell code.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>