submodule: teach set-url subcommand

submodule: teach set-url subcommand

Currently, in the event that a submodule's upstream URL changes, users
have to manually alter the URL in the .gitmodules file then run
`git submodule sync`. Let's make that process easier.

Teach submodule the set-url subcommand which will automatically change
the `submodule.$name.url` property in the .gitmodules file and then run
`git submodule sync` to complete the process.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>