doc: filter-branch does not require re-export of vars

The function `set_ident` in `filter-branch` exported the variables
GIT_(AUTHOR|COMMITTER)_(NAME|EMAIL|DATE) at least since 6f6826c in 2007.
Therefore the filter scripts don't need to re-eport them again.

Signed-off-by: Andreas Heiduk <asheiduk@gmail.com>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: asheiduk

Documentation/git-filter-branch.txt

@@ -86,8 +86,7 @@ OPTIONS
 	This filter may be used if you only need to modify the environment
 	in which the commit will be performed.  Specifically, you might
 	want to rewrite the author/committer name/email/time environment
-	variables (see linkgit:git-commit-tree[1] for details).  Do not forget
-	to re-export the variables.
+	variables (see linkgit:git-commit-tree[1] for details).
 
 --tree-filter <command>::
 	This is the filter for rewriting the tree and its contents.
@@ -342,12 +341,10 @@ git filter-branch --env-filter '
 	if test "$GIT_AUTHOR_EMAIL" = "root@localhost"
 	then
 		GIT_AUTHOR_EMAIL=john@example.com
-		export GIT_AUTHOR_EMAIL
 	fi
 	if test "$GIT_COMMITTER_EMAIL" = "root@localhost"
 	then
 		GIT_COMMITTER_EMAIL=john@example.com
-		export GIT_COMMITTER_EMAIL
 	fi
 ' -- --all
 --------------------------------------------------------