fix: report a missing repository path instead of a traceback

[itzzdev09]

Running gitstats on a path that does not exist ends in a traceback from os.chdir:

$ gitstats ./typo out
  File ".../gitstats/main.py", in _run_single_repo
    os.chdir(gitpath)
FileNotFoundError: [Errno 2] No such file or directory: './typo'

_run_single_repo already raises RuntimeError for a bad output path, which run() reports as a FATAL message with exit code 1 and the multi-repository mode lists as a failed repository. It now does the same for a git path that is not a directory:

$ gitstats ./typo out
FATAL: Git path is not a directory: ./typo

Added test_run_missing_repository_path, which fails on main. The full suite passes and ruff check is clean.

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📚 Documentation preview 📚: https://gitstats--286.org.readthedocs.build/

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[shenxianpeng]

looks good to me. thanks!