git-submodule.sh: improve parsing of some long options

git-submodule.sh: improve parsing of some long options

Some command-line options have a long form which takes an argument. In
this case, the argument can be given right after `='; for example,
"--depth" takes a numerical argument, which can be given as "--depth=X".

Support the case where the argument is given right after `=' for all
long options, in order to improve consistency throughout the script.

Signed-off-by: Roy Eldar <royeldar0@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>