bug#15234: 24.3.50; Make C-x u repeatable

[Jambunathan K]

Make C-x u repeatable.  This will make editing (and completion)
experience quite awesome.  If something is preventing, atleast make it a
customizable option.

| Keyseq 1 => | C-x u | C-x u | C-x u | C-x u | u   |   |
|-------------+-------+-------+-------+-------+-----+---|
| Keyseq 2 => | C-x u | C-x z | z     | z     | C-g | u |
|-------------+-------+-------+-------+-------+-----+---|
| keyseq 3 => | C-x   | u     | u     | u     | C-g | u |


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When I do, M-/ (runs the stock `dabbrev-expand') - this I use very
frequently - I often overshoot the completion that I expect by miles and
use (C-x u)+ to backtrack. 

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In GNU Emacs 24.3.50.3 (i686-pc-linux-gnu, GTK+ Version 2.20.1)
 of 2013-08-31 on porunai
Bzr revision: 114077 rgm@gnu.org-20130830174039-3aiddsbwhbn5tf9x
Windowing system distributor `The X.Org Foundation', version 11.0.10707000
Important settings:
  value of $LANG: en_IN
  locale-coding-system: iso-latin-1-unix
  default enable-multibyte-characters: t