"git archive" recorded incorrect length in extended pax header in
some corner cases, which has been corrected.

* rs/pax-extended-header-length-fix:
  archive-tar: turn length miscalculation warning into BUG
  archive-tar: use size_t in strbuf_append_ext_header()
  archive-tar: fix pax extended header length calculation
  archive-tar: report wrong pax extended header length