Handle not in git repository without error 500 to avoid confusion

Description

Hi. git extension seems to output a rather ugly error message in the bash console when the current working directory is not a git repository, or when switching to such a directory:

With debug mode on, the error seems to be triggered by the following:

[D 20:39:53.173 LabApp.jupyterlab_git] Execute ['git', 'rev-parse', '--show-toplevel'] in /opt/files/jyusr.
[D 20:39:53.187 LabApp.jupyterlab_git] Code: 128
    Output: 
    Error: fatal: not a git repository (or any of the parent directories): .git

which is followed by the following (showing in the console even without debug mode):

[E 20:39:53.187 LabApp] {
      "Host": "127.0.0.1:8888",
      "User-Agent": "Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:83.0) Gecko/20100101 Firefox/83.0",
      "Accept": "*/*",
      "Accept-Language": "en-US,en;q=0.5",
      "Accept-Encoding": "gzip, deflate, br",
      "Referer": "https://127.0.0.1:8888/lab?",
      "Content-Type": "text/plain;charset=UTF-8",
      "X-Xsrftoken": "2|0d88b5b9|xxx|1607891023",
      "Origin": "https://127.0.0.1:8888",
      "Content-Length": "24",
      "Connection": "keep-alive",
      "Cookie": "username-127-0-0-1-8888=\"2|1:0|10:1607891028|23:username-127-0-0-1-8888|44:xxx=|xxxx\"; _xsrf=2|0d88b5b9|xxx|1607891023",
      "Pragma": "no-cache",
      "Cache-Control": "no-cache"
    }
[E 20:39:53.188 LabApp] 500 POST /git/show_top_level?1607891993159 (127.0.0.1) 23.65ms referer=https://127.0.0.1:8888/lab?

Am I doing anything wrong or is this expected behavior?

Reproduce

Launch Jupyter Lab. Working directory is not a git repository.

Expected behavior

Should not show anything even if I’m not in a git repo directory.

Context

jupyter                   1.0.0                      py_2    conda-forge
jupyter-lsp               0.9.3                    pypi_0    pypi
jupyter_client            6.1.7                      py_0    conda-forge
jupyter_console           6.2.0                      py_0    conda-forge
jupyter_core              4.7.0            py38h578d9bd_0    conda-forge
jupyterlab                2.2.9                      py_0    conda-forge
jupyterlab-git            0.23.2             pyhd8ed1ab_0    conda-forge
jupyterlab_pygments       0.1.2              pyh9f0ad1d_0    conda-forge
jupyterlab_server         1.2.0                      py_0    conda-forge

gitpython                 3.1.11                     py_0    conda-forge
ipython                   7.19.0           py38h81c977d_0    conda-forge
python                    3.8.6           h852b56e_0_cpython    conda-forge

jupyterlab-git            0.23.2             pyhd8ed1ab_0    conda-forge

• Operating System and its version: Ubuntu 20.04

[fcollonval]

Solved by #855

[jeanmonet]

sorry was a bit late, but here goes PR proposal.