filter-branch documentation: non-zero exit status in command abort th…

…e filter

Since commit 8c1ce0f filter-branch fails
when a <command> has a non-zero exit status. This commit makes it clear
in the documentation and also fixes the parent-filter example, that was
incorrectly returning non-zero when the commit being tested wasn't the
one to be rewritten.

Signed-off-by: Caio Marcelo de Oliveira Filho <cmarcelo@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-filter-branch.txt

@@ -56,7 +56,9 @@ notable exception of the commit filter, for technical reasons).
 Prior to that, the $GIT_COMMIT environment variable will be set to contain
 the id of the commit being rewritten.  Also, GIT_AUTHOR_NAME,
 GIT_AUTHOR_EMAIL, GIT_AUTHOR_DATE, GIT_COMMITTER_NAME, GIT_COMMITTER_EMAIL,
-and GIT_COMMITTER_DATE are set according to the current commit.
+and GIT_COMMITTER_DATE are set according to the current commit. If any
+evaluation of <command> returns a non-zero exit status, the whole operation
+will be aborted.
 
 A 'map' function is available that takes an "original sha1 id" argument
 and outputs a "rewritten sha1 id" if the commit has been already
@@ -197,7 +199,7 @@ happened).  If this is not the case, use:
 
 --------------------------------------------------------------------------
 git filter-branch --parent-filter \
-	'cat; test $GIT_COMMIT = <commit-id> && echo "-p <graft-id>"' HEAD
+	'test $GIT_COMMIT = <commit-id> && echo "-p <graft-id>" || cat' HEAD
 --------------------------------------------------------------------------
 
 or even simpler: