git-submodule module_name: avoid using unwieldy "value_regexp" feature.

git-submodule module_name: avoid using unwieldy "value_regexp" feature.

"module_name $path" function wants to look up a configuration
variable "submodule.<modulename>.path" whose value is $path, and
return the <modulename> found.  "git-config --get-regexp" is the
natural thing to use for this, but (1) its value matching has an
unfortunate "feature" that takes leading '!' specially, and (2)
its output needs to be parsed with sed to extract <modulename>
part anyway.

This changes the call to "git-config --get-regexp" not to use
the value-regexp part, and moves the "pick the one whose value
is $path" part to the downstream sed.

Signed-off-by: Junio C Hamano <gitster@pobox.com>