templater: fix precedence of --style and --template options

Since e3eb480a9391, --template option is ignored if --style is specified,
which is wrong according to the doc of show_changeset():

    Display format will be the first non-empty hit of:
    1. option 'template'
    2. option 'style'
    ...

--HG--
branch : stable
extra : rebase_source : 26a27e6bbf3bc73558faba5ba1f6e56e7413027b