filter-branch documentation: clarify which filters are eval'ed

All filters, except the commit filter, are evaluated.

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

Documentation/git-filter-branch.txt

@@ -50,7 +50,8 @@ Filters
 ~~~~~~~
 
 The filters are applied in the order as listed below.  The <command>
-argument is always evaluated in shell using the 'eval' command.
+argument is always evaluated in shell using the 'eval' command (with the
+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,