test-lib: limit the output of the yes utility

test-lib: limit the output of the yes utility

On Windows, there is no SIGPIPE. A consequence of this is that the
upstream process of a pipe does not notice the death of the downstream
process until the pipe buffer is full and writing more data returns an
error. This behavior is the reason for an annoying delay during the
execution of t7610-mergetool.sh: There are a number of test cases where
'yes' is invoked upstream. Since the utility is basically an endless
loop it runs, on Windows, until the pipe buffer is full. This does take
a few seconds.

The test suite has its own implementation of 'yes'. Modify it to produce
only a limited amount of output that is sufficient for the test suite.
The amount chosen should be sufficiently high for any test case, assuming
that future test cases will not exaggerate their demands of input from
an upstream 'yes' invocation.

[j6t: commit message]

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