ident: add the ability to provide a "fallback identity"

ident: add the ability to provide a "fallback identity"

In 3bc2111fc2e9 (stash: tolerate missing user identity, 2018-11-18),
`git stash` learned to provide a fallback identity for the case that no
proper name/email was given (and `git stash` does not really care about
a correct identity anyway, but it does want to create a commit object).

In preparation for the same functionality in the upcoming built-in
version of `git stash`, let's offer the same functionality as an API
function.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
[tg: add docs; make it a bug to call the function before other
functions in the ident API]
Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>