git-p4: yes/no prompts should sanitize user text

git-p4: yes/no prompts should sanitize user text

When prompting the user interactively for direction, the tests are
not forgiving of user input format.

For example, the first query asks for a yes/no response. If the user
enters the full word "yes" or "no" or enters a capital "Y" the test
will fail.

Create a new function, prompt(prompt_text) where
  * prompt_text is the text prompt for the user
  * returns a single character where valid return values are
      found by inspecting prompt_text for single characters
      surrounded by square brackets

This new function must  prompt the user for input and sanitize it by
converting the response to a lower case string, trimming leading and
trailing spaces, and checking if the first character is in the list
of choices. If it is, return the first letter.

Change the current references to raw_input() to use this new function.

Since the method requires the returned text to be one of the available
choices, remove the loop from the calling code that handles response
verification.

Thanks-to: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Ben Keene <seraphire@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>