git-stash.sh: don't default to refs/stash if invalid ref supplied

apply_stash() and show_stash() each call rev-parse with
'--default refs/stash' as an argument. This option causes rev-parse to
operate on refs/stash if it is not able to successfully operate on any
element of the command line. This includes failure to supply a "valid"
revision. This has the effect of causing 'stash apply' and 'stash show'
to operate as if stash@{0} had been supplied when an invalid revision is
supplied.

e.g. 'git stash apply stash@{1}' would fall back to
     'git stash apply stash@{0}'

This patch modifies these two functions so that they avoid using the
--default option of rev-parse.

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

git-stash.sh

@@ -144,7 +144,14 @@ show_stash () {
 	then
 		flags=--stat
 	fi
-	s=$(git rev-parse --revs-only --no-flags --default $ref_stash "$@")
+
+	if test $# = 0
+	then
+		set x "$ref_stash@{0}"
+		shift
+	fi
+
+	s=$(git rev-parse --revs-only --no-flags "$@")
 
 	w_commit=$(git rev-parse --verify "$s") &&
 	b_commit=$(git rev-parse --verify "$s^") &&
@@ -163,13 +170,19 @@ apply_stash () {
 		shift
 	esac
 
+	if test $# = 0
+	then
+		set x "$ref_stash@{0}"
+		shift
+	fi
+
 	# current index state
 	c_tree=$(git write-tree) ||
 		die 'Cannot apply a stash in the middle of a merge'
 
 	# stash records the work tree, and is a merge between the
 	# base commit (first parent) and the index tree (second parent).
-	s=$(git rev-parse --revs-only --no-flags --default $ref_stash "$@") &&
+	s=$(git rev-parse --revs-only --no-flags "$@") &&
 	w_tree=$(git rev-parse --verify "$s:") &&
 	b_tree=$(git rev-parse --verify "$s^1:") &&
 	i_tree=$(git rev-parse --verify "$s^2:") ||