Added a check for stash before executing git stash list for performance

[82phil]

Background

I noticed that when in Git repositories the prompt would delay for roughly a half a second before returning.

Running WSL1 on the Windows 1909 release, other version details:

$ git --version
git version 2.17.1
$ lsb_release -a
…
Description:    Ubuntu 18.04.1 LTS

Also somewhat discussed in issue #129 - Latency

Investigation

I walked segment-git.go and tried out the commands it would issue. That is where I found that git stash list takes considerably longer.

Diving into the source for Git (for the 2.17 release) I found that git stash would call a shell script that added overhead. I also found the script provides a quick method to check if a stash existed, see https://github.com/git/git/blob/6e9e91e9cae74cd7feb9300563d40361b2b17dd2/git-stash.sh#L379

Changes

I simply added the check for a stash. The command raises an error if there is no stash.

This also means that if a stash exists, performance will regress