Sync with 1.8.1 maintenance track

Sync with 1.8.1 maintenance track

* maint-1.8.1:
  Start preparing for 1.8.1.6
  git-tag(1): we tag HEAD by default
  Fix revision walk for commits with the same dates
  t2003: work around path mangling issue on Windows
  pack-refs: add fully-peeled trait
  pack-refs: write peeled entry for non-tags
  use parse_object_or_die instead of die("bad object")
  avoid segfaults on parse_object failure
  entry: fix filter lookup
  t2003: modernize style
  name-hash.c: fix endless loop with core.ignorecase=true