submodule-config: rename commit_sha1 to treeish_name

submodule-config: rename commit_sha1 to treeish_name

It is also possible to pass in any treeish name to lookup a submodule
config. Make it clear by naming the variables accordingly. Looking up
a submodule config by tree hash will come in handy in a later patch.

Signed-off-by: Stefan Beller <sbeller@google.com>
Reviewed-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>