project: add basic path checks for <copyfile> & <linkfile>

Reject paths in <copyfile> & <linkfile> that try to use symlinks or
non-file or non-dirs.

We don't fully validate <linkfile> when src is a glob as it's a bit
complicated -- any component in the src could be the glob.  We make
sure the destination is a directory, and that any paths in that dir
are created as symlinks.  So while this can be used to read any path,
it can't be abused to write to any paths.

Bug: https://crbug.com/gerrit/11218
Change-Id: I68b6d789b5ca4e43f569e75e8b293b3e13d3224b
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/233074


Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
Reviewed-by: Michael Mortensen <mmortensen@google.com>