help: use --template to specify existing style

--style is deprecated since 3a35ba2681ec and 870d60294b04.

--HG--
branch : stable
extra : rebase_source : 8fe17fe29b5c4cccc6df6eb33e357537f673321e
templates: re-add template listing support

We used to have --style nosuch to list templates, but --style is now
merged with --template/-T where random strings are acceptable
templates. So we reserve 'list' to allow listing templates.

--HG--
branch : stable
extra : rebase_source : 8696e5260220644304a2d9d9adb63bf3ce4a543d
help: drop reference to glog in templates topic

--HG--
branch : stable
extra : rebase_source : 443ea96d5ec7e615707fa6138f5fb4ce9327e64c
help: mention '-T list' in templater topic

--HG--
branch : stable
extra : rebase_source : c9019feedea5c99783cd216d15c6b3423573f466
extra : amend_source : f87f5d6b5ca415855f0571d36259a737464845d2