Warn user about long git diff

[Egor-S]

Closes #456

• Warn user if git diff takes longer than 5 seconds
  • Suggest updating .gitignore
• Gracefully exit from git diff on Ctrl+C
• Do not serialize repo_diff
  • It accelerates get_run_plan for large diffs

[Egor-S]

Ctrl+C may be ignored during subprocess deletion. However, it is not very likely and doesn't affect the consistency

Exception ignored in: <function Git.AutoInterrupt.__del__ at 0x10e8988c0>
Traceback (most recent call last):
  File "/Users/egors/dstack/envs/p37/lib/python3.7/site-packages/git/cmd.py", line 567, in __del__
    self._terminate()