filter-branch: prevent filters from reading from stdin

stdin is the list of commits when the env, tree and index
filter are executed.  The filters are not supposed to read
anything from stdin so the best is to give them /dev/null
for reading.

Signed-off-by: Matthias Lederhofer <matled@gmx.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-filter-branch.sh

@@ -349,21 +349,21 @@ while read commit; do
 
 	eval "$(set_ident AUTHOR <../commit)"
 	eval "$(set_ident COMMITTER <../commit)"
-	eval "$filter_env"
+	eval "$filter_env" < /dev/null
 
 	if [ "$filter_tree" ]; then
 		git-checkout-index -f -u -a
 		# files that $commit removed are now still in the working tree;
 		# remove them, else they would be added again
 		git-ls-files -z --others | xargs -0 rm -f
-		eval "$filter_tree"
+		eval "$filter_tree" < /dev/null
 		git-diff-index -r $commit | cut -f 2- | tr '\n' '\0' | \
 			xargs -0 git-update-index --add --replace --remove
 		git-ls-files -z --others | \
 			xargs -0 git-update-index --add --replace --remove
 	fi
 
-	eval "$filter_index"
+	eval "$filter_index" < /dev/null
 
 	parentstr=
 	for parent in $(get_parents $commit); do