Merge branch 'ps/mv-contradiction-fix'

"git mv a a/b dst" would ask to move the directory 'a' itself, as well as its contents, in a single destination directory, which is a contradicting request that is impossible to satisfy. This case is now detected and the command errors out. * ps/mv-contradiction-fix: builtin/mv: convert assert(3p) into `BUG()` builtin/mv: bail out when trying to move child and its parent