completion: handle unstuck form of base git options

git-completion.bash's parsing of the command name relies on everything
preceding it starting with '-' unless it is the "-c" option.  This
allows users to use the stuck form of "--work-tree=<path>" and
"--namespace=<path>" but not the unstuck forms "--work-tree <path>" and
"--namespace <path>".  Fix this.

Similarly, the completion only handles the stuck form "--git-dir=<path>"
and not "--git-dir <path>", so fix this as well.

Signed-off-by: John Keeping <john@keeping.me.uk>
completion: learn more git base options

Signed-off-by: John Keeping <john@keeping.me.uk>