ident: die on bogus date format

If the user gives "git commit --date=foobar", we silently ignore the --date flag. We should note the error. This patch puts the fix at the lowest level of fmt_ident, which means it also handles GIT_AUTHOR_DATE=foobar, as well. There are two down-sides to this approach: 1. Technically this breaks somebody doing something like "git commit --date=now", which happened to work because bogus data is the same as "now". Though we do explicitly handle the empty string, so anybody passing an empty variable through the environment will still work. If the error is too much, perhaps it can be downgraded to a warning? 2. The error checking happens _after_ the commit message is written, which can be annoying to the user. We can put explicit checks closer to the beginning of git-commit, but that feels a little hack-ish; suddenly git-commit has to care about how fmt_ident works. Maybe we could simply call fmt_ident earlier? Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>