Fix git pull for external_components

[mmakaay]

What does this implement/fix?

Some refactoring was done on the external_components code between v1.19.4 and v1.20.0, which broke the git pull that is done when the refresh time has passed. The error that now pops up looks like:

external_components: [source xxxxxxxxx.yaml:xxx]
  - [source xxxxxxxxx.yaml:xxx]
    
    not a git repository (or any of the parent directories): .git.                                             

The underlying issue is that the subprocess that is used for running various git commands does not use the directory where the external component is cloned as its working directory.
The change in this pull request fixes the issue.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Other

Related issue or feature (if applicable): fixes

Pull request in esphome-docs with documentation (if applicable): esphome/esphome-docs#

Test Environment

• ESP32
• ESP8266

Example entry for config.yaml:

# Example config.yaml
external_components:
  - source:
      type: git
      url: https://github.com/mmakaay/esphome-xiaomi_bslamp2
      ref: main
    refresh: 1s

Checklist:

• The code change is tested and works locally.
• Tests have been added to verify that the new code works (under tests/ folder).

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in esphome-docs.

[OttoWinter]

Whoops, sorry for the regression. Thanks! 😀