parse-options: replace opterror() with optname()

parse-options: replace opterror() with optname()

Introduce optname() that does the early half of original opterror() to
come up with the name of the option reported back to the user, and use
it to kill opterror().  The callers of opterror() now directly call
error() using the string returned by opterror() instead.

There are a few issues with opterror()

- it tries to assemble an English sentence from pieces. This is not
  great for translators because we give them pieces instead of a full
  sentence.

- It's a wrapper around error() and needs some hack to let the
  compiler know it always returns -1.

- Since it takes a string instead of printf format, one call site has
  to assemble the string manually before passing to it.

Using error() directly solves the second and third problems.

It kind helps the first problem as well because "%s does foo" does
give a translator a full sentence in a sense and let them reorder if
needed. But it has limitations, if the subject part has to change
based on the rest of the sentence, that language is screwed. This is
also why I try to avoid calling optname() when 'flags' is known in
advance.

Mark of these strings for translation as well while at there.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>