combine-packs.sh: update to latest
Exits with failure if any input objects are not present in the current
git directory without the new --ignore-missing-objects option.
Supports an objects subdirectory that is a symlink to the real location.
Supports the GIT_OBJECT_DIRECTORY environment variable (but only if the
new --envok option is given).
Shows the exit status of any failed command.
Uses -t " " with sort to correctly match join usage.
Avoids SIGPIPE on sort output piped to join -.
Suppresses unwanted extraneous objects when isolated trees are present.
Orders commits properly rather than using input order.
Avoids "exec command ..." as some shells screw that up.
Signed-off-by: Kyle J. McKay <mackyle@gmail.com>