Refactor patch_update_cmd

Refactor patch_update_cmd

Split patch_update_cmd into two functions, one to prompt the user for
a path to patch and another to do the actual work given that file path.
This lays the groundwork for a future commit which will teach
git-add--interactive to accept a path parameter and jump directly to
the patch subcommand for that path, bypassing the interactive prompt.

Signed-off-by: Wincent Colaiuta <win@wincent.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>