bash completion: refactor common log, shortlog and gitk options

Refactor options that are useful for more than one of them into a
variable used by the relevant completions.  This has the effect of
adding the following options to git-log:

--branches --tags --remotes --first-parent --dense --sparse
--simplify-merges --simplify-by-decoration --first-parent
--no-merges

The following to git-shortlog:

--branches --tags --remotes --first-parent

And the following to gitk:

--branches --tags --remotes --first-parent --no-merges --max-count=
--max-age= --since= --after= --min-age= --until= --before= --dense
--sparse --full-history --simplify-merges --simplify-by-decoration
--left-right

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Acked-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>