rebase-interactive: drop early check for valid ident

rebase-interactive: drop early check for valid ident

Since the very inception of interactive-rebase in 1b1dce4
(Teach rebase an interactive mode, 2007-06-25), there has
been a preemptive check, before looking at any commits, to
see whether the user has a valid name/email combination.

This is convenient, because it means that we abort the
operation before even beginning (rather than just
complaining that we are unable to pick a particular commit).

However, it does the wrong thing when the rebase does not
actually need to generate any new commits (e.g., a
fast-forward with no commits to pick, or one where the base
stays the same, and we just pick the same commits without
rewriting anything). In this case it may complain about the
lack of ident, even though one would not be needed to
complete the operation.

This may seem like mere nit-picking, but because interactive
rebase underlies the "preserve-merges" rebase, somebody who
has set "pull.rebase" to "preserve" cannot make even a
fast-forward pull without a valid ident, as we bail before
even realizing the fast-forward nature.

This commit drops the extra ident check entirely. This means
we rely on individual commands that generate commit objects
to complain. So we will continue to notice and prevent cases
that actually do create commits, but with one important
difference: we fail while actually executing the "pick"
operations, and leave the rebase in a conflicted, half-done
state.

In some ways this is less convenient, but in some ways it is
more so; the user can then manually commit or even "git
rebase --continue" after setting up their ident (or
providing it as a one-off on the command line).

Reported-by: Dakota Hawkins <dakotahawkins@gmail.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>