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 <[email protected]>
Signed-off-by: Dirk Gouders <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: dgouders

Documentation/MyFirstObjectWalk.txt

@@ -848,7 +848,7 @@ those lines without having to recompile.
 With only that change, run again (but save yourself some scrollback):
 
 ----
-$ GIT_TRACE=1 ./bin-wrappers/git walken | head -n 10
+$ GIT_TRACE=1 ./bin-wrappers/git walken 2>&1 | head -n 10
 ----
 
 Take a look at the top commit with `git show` and the object ID you printed; it
@@ -876,7 +876,7 @@ of the first handful:
 
 ----
 $ make
-$ GIT_TRACE=1 ./bin-wrappers git walken | tail -n 10
+$ GIT_TRACE=1 ./bin-wrappers/git walken 2>&1 | tail -n 10
 ----
 
 The last commit object given should have the same OID as the one we saw at the