detached-stash: introduce parse_flags_and_revs function

Introduce parse_flags_and_revs. This function requires that
there is at most one stash-like revision parameter and
zero or more flags.

It knows how to parse -q,--quiet and --index flags, but leaves
other flags parsed.

Specified revisions are checked to see that they are at
least stash-like (meaning: they look like something created
by git stash save or git stash create).

If this is so, then IS_STASH_LIKE is initialized to a non-empty value.

If the specified revision also looks like a stash log entry reference,
then IS_STASH_REF is initialized to a non-empty value.

References of the form ref@{spec} are required to precisely identify
an individual commit.

If no reference is specified, stash@{0} is assumed.

Once the specified reference is validated to be at least stash_like
an ensemble of derived variables, (w_commit, w_tree, b_commit, etc)
is initialized with a single call to git rev-parse.

Repeated calls to parse_flags_and_rev() avoid repeated calls
to git rev-parse if the specified arguments have already been
parsed.

Subsequent patches in the series modify the existing
git stash subcommands to make use of these functions
as appropriate.

An ensemble of supporting functions that make use of the state
established by parse_flags_and_rev(). These are described below:

The ancillary functions are:

is_stash_like(): which can be used to test
whether a specified commit looks like a commit created with
git stash save or git stash create.

assert_stash_like(): which can be used by
commands that misbehave unless their arguments stash-like.

is_stash_ref(): which checks whether an argument
is valid stash reference(e.g. is of the form
['refs/']stash['@{'something'}])

assert_stash_ref(): which can be used by commands
that misbehave unless their arguments are both stash-like and
refer to valid stash entries.

Signed-off-by: Jon Seymour <jon.seymour@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-stash.sh

@@ -225,6 +225,129 @@ show_stash () {
 	git diff $flags $b_commit $w_commit
 }
 
+#
+# Parses the remaining options looking for flags and
+# at most one revision defaulting to ${ref_stash}@{0}
+# if none found.
+#
+# Derives related tree and commit objects from the
+# revision, if one is found.
+#
+# stash records the work tree, and is a merge between the
+# base commit (first parent) and the index tree (second parent).
+#
+#   REV is set to the symbolic version of the specified stash-like commit
+#   IS_STASH_LIKE is non-blank if ${REV} looks like a stash
+#   IS_STASH_REF is non-blank if the ${REV} looks like a stash ref
+#   s is set to the SHA1 of the stash commit
+#   w_commit is set to the commit containing the working tree
+#   b_commit is set to the base commit
+#   i_commit is set to the commit containing the index tree
+#   w_tree is set to the working tree
+#   b_tree is set to the base tree
+#   i_tree is set to the index tree
+#
+#   GIT_QUIET is set to t if -q is specified
+#   INDEX_OPTION is set to --index if --index is specified.
+#   FLAGS is set to the remaining flags
+#
+# dies if:
+#   * too many revisions specified
+#   * no revision is specified and there is no stash stack
+#   * a revision is specified which cannot be resolve to a SHA1
+#   * a non-existent stash reference is specified
+#
+
+parse_flags_and_rev()
+{
+	test "$PARSE_CACHE" = "$*" && return 0 # optimisation
+	PARSE_CACHE="$*"
+
+	IS_STASH_LIKE=
+	IS_STASH_REF=
+	INDEX_OPTION=
+	s=
+	w_commit=
+	b_commit=
+	i_commit=
+	w_tree=
+	b_tree=
+	i_tree=
+
+	REV=$(git rev-parse --no-flags --symbolic "$@" 2>/dev/null)
+	FLAGS=$(git rev-parse --no-revs -- "$@" 2>/dev/null)
+
+	set -- $FLAGS
+
+	FLAGS=
+	while test $# -ne 0
+	do
+		case "$1" in
+			-q|--quiet)
+				GIT_QUIET=-t
+			;;
+			--index)
+				INDEX_OPTION=--index
+			;;
+			--)
+				:
+			;;
+			*)
+				FLAGS="${FLAGS}${FLAGS:+ }$1"
+			;;
+		esac
+		shift
+	done
+
+	set -- $REV
+
+	case $# in
+		0)
+			have_stash || die "No stash found."
+			set -- ${ref_stash}@{0}
+		;;
+		1)
+			:
+		;;
+		*)
+			die "Too many revisions specified: $REV"
+		;;
+	esac
+
+	REV=$(git rev-parse --quiet --symbolic --verify $1 2>/dev/null) || die "$1 is not valid reference"
+
+	i_commit=$(git rev-parse --quiet --verify $REV^2 2>/dev/null) &&
+	set -- $(git rev-parse $REV $REV^1 $REV: $REV^1: $REV^2: 2>/dev/null) &&
+	s=$1 &&
+	w_commit=$1 &&
+	b_commit=$2 &&
+	w_tree=$3 &&
+	b_tree=$4 &&
+	i_tree=$5 &&
+	IS_STASH_LIKE=t &&
+	test "$ref_stash" = "$(git rev-parse --symbolic-full-name "${REV%@*}")" &&
+	IS_STASH_REF=t
+
+}
+
+is_stash_like()
+{
+	parse_flags_and_rev "$@"
+	test -n "$IS_STASH_LIKE"
+}
+
+assert_stash_like() {
+	is_stash_like "$@" || die "'$*' is not a stash-like commit"
+}
+
+is_stash_ref() {
+	is_stash_like "$@" && test -n "$IS_STASH_REF"
+}
+
+assert_stash_ref() {
+	is_stash_ref "$@" || die "'$*' is not a stash reference"
+}
+
 apply_stash () {
 	applied_stash=
 	unstash_index=
@@ -375,6 +498,7 @@ apply_to_branch () {
 	drop_stash $stash
 }
 
+PARSE_CACHE='--not-parsed'
 # The default command is "save" if nothing but options are given
 seen_non_option=
 for opt