pathspec: give better message for submodule related pathspec error

pathspec: give better message for submodule related pathspec error

Every once in a while someone complains to the mailing list to have
run into this weird assertion[1]. The usual response from the mailing
list is link to old discussions[2], and acknowledging the problem
stating it is known.

This patch accomplishes two things:

  1. Switch assert() to die("BUG") to give a more readable message.

  2. Take one of the cases where we hit a BUG and turn it into a normal
     "there was something wrong with the input" message.

     This assertion triggered for cases where there wasn't a programming
     bug, but just bogus input. In particular, if the user asks for a
     pathspec that is inside a submodule, we shouldn't assert() or
     die("BUG"); we should tell the user their request is bogus.

     The only reason we did not check for it, is the expensive nature
     of such a check, so callers avoid setting the flag
     PATHSPEC_STRIP_SUBMODULE_SLASH_EXPENSIVE. However when we die due
     to bogus input, the expense of CPU cycles spent outweighs the user
     wondering what went wrong, so run that check unconditionally before
     dying with a more generic error message.

Note: There is a case (e.g. "git -C submodule add .") in which we call
strip_submodule_slash_expensive, as git-add requests it via the flag
PATHSPEC_STRIP_SUBMODULE_SLASH_EXPENSIVE, but the assert used to
trigger nevertheless, because the flag PATHSPEC_LITERAL was not set,
such that we executed

	if (item->nowildcard_len < prefixlen)
		item->nowildcard_len = prefixlen;

and prefixlen was not adapted (e.g. it was computed from "submodule/")
So in the die_inside_submodule_path function we also need handle paths,
that were stripped before, i.e. are the exact submodule path. This
is why the conditions in die_inside_submodule_path are slightly
different than in strip_submodule_slash_expensive.

[1] https://www.google.com/search?q=item-%3Enowildcard_len
[2] http://git.661346.n2.nabble.com/assert-failed-in-submodule-edge-case-td7628687.html
    https://www.spinics.net/lists/git/msg249473.html

Helped-by: Jeff King <peff@peff.net>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>