t3905: remove nested git in command substitution

If a git command in a nested command substitution fails, it will be
silently ignored since only the return code of the outer command
substitutions is reported. Factor out nested command substitutions so
that the error codes of those commands are reported.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3905-stash-include-untracked.sh

@@ -36,8 +36,10 @@ test_expect_success 'stash save --include-untracked cleaned the untracked files'
 '
 
 test_expect_success 'stash save --include-untracked stashed the untracked files' '
-	tracked=$(git rev-parse --short $(echo 1 | git hash-object --stdin)) &&
-	untracked=$(git rev-parse --short $(echo untracked | git hash-object --stdin)) &&
+	one_blob=$(echo 1 | git hash-object --stdin) &&
+	tracked=$(git rev-parse --short "$one_blob") &&
+	untracked_blob=$(echo untracked | git hash-object --stdin) &&
+	untracked=$(git rev-parse --short "$untracked_blob") &&
 	cat >expect.diff <<-EOF &&
 	diff --git a/HEAD b/HEAD
 	new file mode 100644
@@ -117,7 +119,8 @@ test_expect_success 'stash save -u dirty index' '
 '
 
 test_expect_success 'stash save --include-untracked dirty index got stashed' '
-	blob=$(git rev-parse --short $(echo 4 | git hash-object --stdin)) &&
+	four_blob=$(echo 4 | git hash-object --stdin) &&
+	blob=$(git rev-parse --short "$four_blob") &&
 	cat >expect <<-EOF &&
 	diff --git a/file3 b/file3
 	new file mode 100644