Merge branch 'mh/path-max'

Merge branch 'mh/path-max'

A few places where we relied on a fixed length buffer to hold
pathnames in these two programs have been converted to use strbuf.

* mh/path-max:
  builtin/prune.c: use strbuf to avoid having to worry about PATH_MAX
  prune-packed: use strbuf to avoid having to worry about PATH_MAX