stack: remove unnecessary prefix from stack output with non-default --template "index" template keyword already exists (the current iteration of the loop), so "stack_index" it is. It follows the same convention as other template keywords, such as "files_added", "line_number", etc.

. $TESTDIR/testlib/pythonpath.sh

cat >> $HGRCPATH << EOF
[extensions]
docgraph=
EOF