If "git pull --recurse-submodules --rebase" is invoked when the current
branch and its corresponding remote-tracking branch have no merge base,
a "bad object" fatal error occurs. This issue was introduced with commit
a6d7eb2c ("pull: optionally rebase submodules (remote submodule
changes only)", 2017-06-23), which also introduced this feature.

This is because cmd_pull() in builtin/pull.c thus invokes
submodule_touches_in_range() with a null OID as the first parameter.
Ensure that this case works, and document what happens in this case.
Signed-off-by: NJonathan Tan <jonathantanmy@google.com>
Reviewed-by: NStefan Beller <sbeller@google.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>