git stash: Give friendlier errors when there is nothing to apply

The change makes sure a stash (given or default) exists before
checking if the working tree is dirty.

If the default stash is requested, the old message was scary and
included a 'fatal' error from rev-parse:
     fatal: Needed a single revision
     : no valid stashed state found

It is replaced with a friendlier 'Nothing to apply' error, similar to
'git stash branch'.

If a specific stash is specified, the 'Needed a single revision' errors
from rev-parse are suppressed.

Signed-off-by: Ori Avtalion <ori@avtalion.name>
Acked-by: Thomas Rast <trast@student.ethz.ch>
Acked-by: Nanako Shiraishi <nanako3@lavabit.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-stash.sh

@@ -162,10 +162,6 @@ show_stash () {
 }
 
 apply_stash () {
-	git update-index -q --refresh &&
-	git diff-files --quiet --ignore-submodules ||
-		die 'Cannot apply to a dirty working tree, please stage your changes'
-
 	unstash_index=
 
 	while test $# != 0
@@ -184,18 +180,27 @@ apply_stash () {
 		shift
 	done
 
-	# current index state
-	c_tree=$(git write-tree) ||
-		die 'Cannot apply a stash in the middle of a merge'
+	if test $# = 0
+	then
+		have_stash || die 'Nothing to apply'
+	fi
 
 	# 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 --verify --default $ref_stash "$@") &&
-	w_tree=$(git rev-parse --verify "$s:") &&
-	b_tree=$(git rev-parse --verify "$s^1:") &&
-	i_tree=$(git rev-parse --verify "$s^2:") ||
+	s=$(git rev-parse --quiet --verify --default $ref_stash "$@") &&
+	w_tree=$(git rev-parse --quiet --verify "$s:") &&
+	b_tree=$(git rev-parse --quiet --verify "$s^1:") &&
+	i_tree=$(git rev-parse --quiet --verify "$s^2:") ||
 		die "$*: no valid stashed state found"
 
+	git update-index -q --refresh &&
+	git diff-files --quiet --ignore-submodules ||
+		die 'Cannot apply to a dirty working tree, please stage your changes'
+
+	# current index state
+	c_tree=$(git write-tree) ||
+		die 'Cannot apply a stash in the middle of a merge'
+
 	unstashed_index_tree=
 	if test -n "$unstash_index" && test "$b_tree" != "$i_tree" &&
 			test "$c_tree" != "$i_tree"