pre-commit: fails for multi-file patches

[casperdcl]

• pre-commit fails when there are more files to check than hyperthreads on the machine
• error message about .dvc/tmp/lock is misleading and uphelpful

Steps to reproduce:

dvc (master)$ pre-commit run --all-files
black....................................................................Passed
flake8...................................................................Passed
beautysh.................................................................Passed
DVC pre-commit...........................................................Failed
- hook id: dvc-pre-commit
- duration: 6.17s
- exit code: 1

ERROR: failed to obtain data status - cannot perform the command because another DVC process seems to be running on this project. If that is not the case, manually remove `.dvc/tmp/lock` and try again.

Having any troubles? Hit us up at https://dvc.org/support, we are always happy to help!

• from lint: isort pre-commit & misc improvements #3749 (comment)

require_serial: true in the pre-commit config doesn't seem to help

[efiop]

Another user experiencing this https://opendatascience.slack.com/archives/CGGLZJ119/p1589988401138600 . Escalating the priority.

[casperdcl]

updated description a bit.