ENH: asv gh-pages custom commit message

[ev-br]

Would be nice if asv gh-pages could grow a cmdline argument to specify a custom commit message to override the default "Generated from sources".

[HaoZeke]

While this would be easy enough to add, gh-pages is one of those sub-commands which should probably be deprecated and become documentation only..

[ev-br]

Deprecating it works too --- either an update or a deprecation would be an improvement IMO.