builtin/repo: add progress meter for structure stats

builtin/repo: add progress meter for structure stats

When using the structure subcommand for git-repo(1), evaluating a
repository may take some time depending on its shape. Add a progress
meter to provide feedback to the user about what is happening. The
progress meter is enabled by default when the command is executed from a
tty. It can also be explicitly enabled/disabled via the --[no-]progress
option.

Signed-off-by: Justin Tobler <jltobler@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>