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

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>