Jeff King recently reinstated -p to suppress the default diffstat
(as -p used to work before 68daa64d, about 14 months ago).

However, -p is also needed in combination with certain options
(e.g. --stat or --numstat) in order to produce any patch at all.
The documentation does not mention this.

Since the purpose of format-patch is to produce a patch that
can be emailed, it does not make sense that certain combination
of options will suppress the generation of the patch itself.

Therefore:

* Update 'git format-patch' to always generate a patch.

* Since the --name-only, --name-status, and --check suppresses
  the generation of the patch, disallow those options,
  and remove the description of them in the documentation.

* Remove the reference to -p in the description of -U.

* Remove the descriptions of the options that are synonyms for -p
  plus another option (--patch-with-raw and --patch-with-stat).

* While at it, slightly tweak the description of -p itself
  to say that it generates "plain patches", so that you can
  think of -p as "plain patch" as an mnemonic aid.
Signed-off-by: NBjörn Gustavsson <bgustavsson@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>