Add API to specify the submodule name

Add API to specify the submodule name

Currently SubmoduleAddCommand always uses the path as submodule name.
This patch lets the caller specify a submodule name.

SubmoduleUpdateCommand still does not make use of the submodule name
(see bug 535027) but Git does.  To avoid triggering CVE-2018-11235,
do some validation on the name to avoid '..' path components.

[jn: fleshed out commit message, mostly to work around flaky CI]

Change-Id: I6879c043c6d7973556e2080387f23c246e3d76a5
Signed-off-by: Masaya Suzuki <masayasuzuki@google.com>
Signed-off-by: Jonathan Nieder <jrn@google.com>