Support --strategy=x completion in addition to --strategy x.

Support --strategy=x completion in addition to --strategy x.

Because git-merge and git-rebase both accept -s, --strategy or --strategy=
we should recognize all three formats in the bash completion functions and
issue back all merge strategies on demand.

I also moved the prior word testing to be before the current word testing,
as the current word cannot be completed with -- if the prior word was an
option which requires a parameter, such as -s or --strategy.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>