submodule: use "fetch" logic instead of custom remote discovery
Replace a use of the get_default_remote() function with an invocation
of "git fetch"
The "fetch" command already has logic to discover the remote for the
current branch. However, before it learned to accept a custom
refspec *and* use its idea of the default remote, it wasn't possible
to get rid of some equivalent of the "get_default_remote" invocation
here.
As it turns out the recently added "--stdin" option to fetch[1] gives
us a way to do that. Let's use it instead.
While I'm at it simplify the "fetch_in_submodule" function. It wasn't
necessary to pass "$@" to "fetch" since we'd only ever provide one
SHA-1 as an argument in the previous "*" codepath (in addition to
"--depth=N"). Rewrite the function to more narrowly reflect its
use-case.