Teach bash completion about 'git remote update'

Teach bash completion about 'git remote update'

Recently the git-remote command grew an update subcommand, which
can be used to execute git-fetch across multiple repositories
in a single step.  These can be configured with the 'remotes.*'
configuration options, so we can offer completion for any name that
matches and appears to be useful to git-remote update.

Signed-off-by: Shawn O. Pearce <redacted>