Merge branch 'ps/mv-contradiction-fix'

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