doc: avoid [<directory>] arg for git-clone(1)

doc: avoid [<directory>] arg for git-clone(1)

While it is possible to host source code from the root of a URL
using git-http-backend(1), the lack of pathname in the URL can
also be confusing to users.  So just add the path name of the
project into the URL itself so users can invoke "git clone"
with one command-line argument instead of two.

Of course, previously documented URLs continue to work as normal.