hgweb: add z-index for search field tooltip
On graph page, search field tooltip sometimes goes down to the graph area,
where it used to be covered by foreground element of graph entries (.fg)
because they have z-index: 10.
To prevent the tooltip from being covered, z-index: 15 is enough.
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.