Silently ignore gitpython import errors.

[tcdent]

gitpython will not import if git is not installed on the system.

Found this cuz I was testing install on a fresh container.

Big scary in case anyone encounters this:

Traceback (most recent call last):
  File "/root/.local/lib/python3.12/site-packages/git/__init__.py", line 296, in <module>
    refresh()
  File "/root/.local/lib/python3.12/site-packages/git/__init__.py", line 287, in refresh
    if not Git.refresh(path=path):
           ^^^^^^^^^^^^^^^^^^^^^^
  File "/root/.local/lib/python3.12/site-packages/git/cmd.py", line 631, in refresh
    raise ImportError(err)
ImportError: Bad git executable.
The git executable must be specified in one of the following ways:
    - be included in your $PATH
    - be set via $GIT_PYTHON_GIT_EXECUTABLE
    - explicitly set via git.refresh(<full-path-to-git-executable>)

All git commands will error until this is rectified.

This initial message can be silenced or aggravated in the future by setting the
$GIT_PYTHON_REFRESH environment variable. Use one of the following values:
    - quiet|q|silence|s|silent|none|n|0: for no message or exception
    - warn|w|warning|log|l|1: for a warning message (logging level CRITICAL, displayed by default)
    - error|e|exception|raise|r|2: for a raised exception

Example:
    export GIT_PYTHON_REFRESH=quiet


The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/root/.local/bin/agentstack", line 5, in <module>
    from agentstack.main import main
  File "/root/.local/lib/python3.12/site-packages/agentstack/main.py", line 7, in <module>
    from agentstack.cli import (
  File "/root/.local/lib/python3.12/site-packages/agentstack/cli/__init__.py", line 1, in <module>
    from .cli import (
  File "/root/.local/lib/python3.12/site-packages/agentstack/cli/cli.py", line 10, in <module>
    from agentstack import repo
  File "/root/.local/lib/python3.12/site-packages/agentstack/repo.py", line 4, in <module>
    import git
  File "/root/.local/lib/python3.12/site-packages/git/__init__.py", line 298, in <module>
    raise ImportError("Failed to initialize: {0}".format(_exc)) from _exc
ImportError: Failed to initialize: Bad git executable.
The git executable must be specified in one of the following ways:
    - be included in your $PATH
    - be set via $GIT_PYTHON_GIT_EXECUTABLE
    - explicitly set via git.refresh(<full-path-to-git-executable>)

All git commands will error until this is rectified.

This initial message can be silenced or aggravated in the future by setting the
$GIT_PYTHON_REFRESH environment variable. Use one of the following values:
    - quiet|q|silence|s|silent|none|n|0: for no message or exception
    - warn|w|warning|log|l|1: for a warning message (logging level CRITICAL, displayed by default)
    - error|e|exception|raise|r|2: for a raised exception

Example:
    export GIT_PYTHON_REFRESH=quiet
    ```

[bboynton97]

def dont want big scary