Add environment variable inheritance to git status and branch API calls

[nsingl00]

Several git subprocess calls in the status and branch APIs were not inheriting the parent process environment variables, which could cause git commands to fail when custom environment configurations are needed (e.g., custom PATH, GIT_*
variables, proxy settings, or authentication tokens).

Added env=os.environ.copy() parameter to the following subprocess calls:

• Status API (git.py:status() method):
• Branch API:

Issue: #1416

[github-actions]

👈 Launch a Binder on branch nsingl00/jupyterlab-git/main

[krassowski]

In general the pattern we use in the codebase is:

env = os.environ.copy()
env["GIT_TERMINAL_PROMPT"] = "0"

but I think these commands should not prompt so I think it is fine to merge as-is.