Merge branch 'nd/magic-pathspec-from-root'

Merge branch 'nd/magic-pathspec-from-root'

When giving arguments without "--" disambiguation, object names
that come  earlier on the command line must not be interpretable as
pathspecs and pathspecs that come later on the command line must
not be interpretable as object names.  Tweak the disambiguation
rule so that ":/" (no other string before or after) is always
interpreted as a pathspec, to avoid having to say "git cmd -- :/".

* nd/magic-pathspec-from-root:
  grep: avoid accepting ambiguous revision
  Update :/abc ambiguity check