Also use TAB for completion with org-insert-link.

Also use TAB for completion with org-insert-link.

Before this patch the completion mechanism was this: TAB let's you
complete through link prefixes (gnus: file: bbdb: ...) then RET allows
completion if a completion mechanism is available for the chosen prefix.
Navigating through the history of stored links was a separate process,
available through the up/down M-n/M-p keys.

Now TAB not only completes through link prefixes but also through stored
links.  This behavior matches other Emacs completion mechanisms a bit
more closely.