v5, 8 partsdetached-stash: regularise handling of stash arguments by git stash

This patch introduces a common flags and revision parsing function to the following git stash commands:
 * apply
 * branch
 * pop
 * drop
 * show

With these changes, git stash now:
 * allows non-stash log entry references to passed to 'stash branch' provided they are stash-like
 * relaxes the requirement that a stash log currently entry exists for 'stash show' or 'stash branch' 
 * does not attempt to drop the specified revision if it doesn't look like a stash log entry reference
 * fails 'stash pop' and 'stash drop' early if the specified revision is not a stash log entry reference
 * fails early if more than one stash-like commit is specified
 * fails early if the specified revision is of the form ref@{n} and ref exists, but ref@{n} does not exist
 * reports various error conditions that can occur across multiple commanbds with consistent error messages.

The implementation of several commands is simplified to a lesser or greater degree by taking advantage of the new common parsing and validation function, parse_flags_and_rev().

This revision incorporates feedback and corrections from Johannes Sixt and Junio Hamano.

Jon Seymour (8):
  detached-stash: introduce parse_flags_and_revs function
  detached-stash: simplify stash_apply
  detached-stash: simplify stash_drop
  detached-stash: refactor git stash pop implementation
  detached-stash: simplify git stash branch
  detached-stash: simplify git stash show
  detached-stash: tests of git stash with stash-like arguments
  detached-stash: update Documentation

 Documentation/git-stash.txt |   16 ++-
 git-stash.sh                |  223 ++++++++++++++++++++++++++++---------------
 t/t3903-stash.sh            |  140 +++++++++++++++++++++++++++
 3 files changed, 296 insertions(+), 83 deletions(-)