ident: reject all-crud ident name

ident: reject all-crud ident name

An ident name consisting of only "crud" characters (like
whitespace or punctuation) is effectively the same as an
empty one, because our strbuf_addstr_without_crud() will
remove those characters.

We reject an empty name when formatting a strict ident, but
don't notice an all-crud one because our check happens
before the crud-removal step.

We could skip past the crud before checking for an empty
name, but let's make it a separate code path, for two
reasons. One is that we can give a more specific error
message. And two is that unlike a blank name, we probably
don't want to kick in the fallback-to-username behavior.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>