bitbake: fetch/git: Remove a possible trailing '/' in subpath

bitbake: fetch/git: Remove a possible trailing '/' in subpath

If the subpath parameter to the git fetcher ends with a trailing '/',
 bb.utils.prunedir() will be called on '/'...

Fixes [YOCTO #7620].

(Bitbake master rev: 380a3fb372c8b0a53dd7528562e6e7a222dc76ef)

(Bitbake rev: faffa1c4a4d8353b21a0d359076153da0dc31a05)

Signed-off-by: Anders Darander <anders@chargestorm.se>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>