Optimize the git command used to check stash

[ncs-msmith]

Compare the output of GIT_TRACE=1 git rev-parse refs/stash
with GIT_TRACE=1 git stash list -n 1. Both return the most recent
saved stash, but rev-parse takes less time and fewer calls.

When there is no saved stash, rev-parse will return 'refs/stash',
so we need to filter that out.

[ncs-msmith]

[502640072:~/liquidprompt] feature/optimize-stash-marker+ 11m28s ± time GIT_TRACE=1 git stash list -n 1
02:15:50.425685 git.c:594               trace: exec: 'git-stash' 'list' '-n' '1'
02:15:50.426245 run-command.c:369       trace: run_command: 'git-stash' 'list' '-n' '1'
02:15:50.832859 git.c:594               trace: exec: 'git-sh-i18n--envsubst' '--variables' 'usage: $dashless $USAGE'
02:15:50.833651 run-command.c:369       trace: run_command: 'git-sh-i18n--envsubst' '--variables' 'usage: $dashless $USAGE'
02:15:51.231793 git.c:594               trace: exec: 'git-sh-i18n--envsubst' 'usage: $dashless $USAGE'
02:15:51.232424 run-command.c:369       trace: run_command: 'git-sh-i18n--envsubst' 'usage: $dashless $USAGE'
02:15:51.638833 git.c:371               trace: built-in: git 'rev-parse' '--git-dir'
02:15:51.837774 git.c:371               trace: built-in: git 'rev-parse' '--git-path' 'objects'
02:15:52.245130 git.c:371               trace: built-in: git 'rev-parse' '--show-toplevel'
02:15:52.457231 git.c:371               trace: built-in: git 'rev-parse' '--git-path' 'index'
02:15:52.653917 git.c:371               trace: built-in: git 'config' '--get-colorbool' 'color.interactive'
02:15:52.857751 git.c:371               trace: built-in: git 'config' '--get-color' 'color.interactive.help' 'red bold'
02:15:53.063719 git.c:371               trace: built-in: git 'config' '--get-color' '' 'reset'
02:15:53.270403 git.c:371               trace: built-in: git 'rev-parse' '--verify' '--quiet' 'refs/stash'
02:15:53.468215 git.c:371               trace: built-in: git 'log' '--format=%gd: %gs' '-g' '--first-parent' '-m' '-n' '1' 'refs/stash' '--'
02:15:53.470152 run-command.c:369       trace: run_command: 'less'
02:15:53.470884 run-command.c:228       trace: exec: 'less'
stash@{0}: WIP on feature/optimize-stash-marker: 8ee89eb Optimize the git command used to check stash

real	0m3.257s
user	0m1.010s
sys	0m2.187s

vs

[502640072:~/liquidprompt] feature/optimize-stash-marker+ 14m50s ± time GIT_TRACE=1 git rev-parse refs/stash
02:16:02.152990 git.c:371               trace: built-in: git 'rev-parse' 'refs/stash'
14ed66dee9795c967283f36f11a11896d473bfea

real	0m0.006s
user	0m0.001s
sys	0m0.003s