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>