Since `ABSPATH` is defined and documented to end with a forward slash `/`, this changeset removes the first `/` from strings appended to `ABSPATH` in various files, leading to `//` in the resulting path.
Props TobiasBg, audrasjb, SergeyBiryukov, emanuelx.
Fixes#57074.
See #57071.
git-svn-id: https://develop.svn.wordpress.org/trunk@54872 602fd350-edb4-49c9-b593-d223f7449a82