submodule add: show 'add --dry-run' stderr when aborting

submodule add: show 'add --dry-run' stderr when aborting

Unless --force is specified, 'submodule add' checks if the destination
path is ignored by calling 'git add --dry-run --ignore-missing', and,
if that call fails, aborts with a custom "path is ignored" message (a
slight variant of what 'git add' shows).  Aborting early rather than
letting the downstream 'git add' call fail is done so that the command
exits before cloning into the destination path.  However, in rare
cases where the dry-run call fails for a reason other than the path
being ignored---for example, due to a preexisting index.lock
file---displaying the "ignored path" error message hides the real
source of the failure.

Instead of displaying the tailored "ignored path" message, let's
report the standard error from the dry run to give the caller more
accurate information about failures that are not due to an ignored
path.

For the ignored path case, this leads to the following change in the
error message:

  The following [-path is-]{+paths are+} ignored by one of your .gitignore files:
  <destination path>
  Use -f if you really want to add [-it.-]{+them.+}

The new phrasing is a bit awkward, because 'submodule add' is only
dealing with one destination path.  Alternatively, we could continue
to use the tailored message when the exit code is 1 (the expected
status for a failure due to an ignored path) and relay the standard
error for all other non-zero exits.  That, however, risks hiding the
message of unrelated failures that share an exit code of 1, so it
doesn't seem worth doing just to avoid a clunkier, but still clear,
error message.

Signed-off-by: Kyle Meyer <kyle@kyleam.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>