MyFirstObjectWalk: add stderr to pipe processing

MyFirstObjectWalk: add stderr to pipe processing

In the last chapter of this document, pipes are used in commands to
filter out the first/last trace messages.  But according to git(1),
trace messages are sent to stderr if GIT_TRACE is set to '1', so those
commands do not produce the described results.

Fix this by redirecting stderr to stdout prior to the pipe operator
to additionally connect stderr to stdin of the latter command.

Further, while reviewing the above fix, Kyle Lippincott noticed
a second issue with the second of the examples: a missing slash in the
executable path "./bin-wrappers git".

Add the missing slash.

Helped-by: Kyle Lippincott <spectral@google.com>
Signed-off-by: Dirk Gouders <dirk@gouders.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>