git archive --remote should generate tar.gz format indicated by -o filename

git archive -o name.tar.gz generates a gzipped file without needing an
explicit --format switch.

However, git archive -o name.tar.gz --remote [url] generates a tar
file, which is unexpected, bandwidth-heavier, and additionally in some
cases it's not immediately obvious that this has happened.

git archive -o name.tar.gz --remote [url] --format tar.gz generates a
gzipped file, so there's obviously no limitation with e.g. git-upload-
archive.

Given the above, either git archive or git-upload-archive should apply
the same tar.gz filename heuristic and generate the expected format.

Presumably e.g. tar.xz support when using --remote would be more
problematic since, in the local case, it involves specifying an
arbitrary command.