fix(format): Allow git-attributes to be set to true

[mrmeku]

Prior to this change, if one of the git attributes used to disable format checks was set to a value
of true, it was still formatted since the only
valid syntax was setting the attribute without
any value.

After this change the attributes can be used without a value or with the value of true

Fixes #383

---

Changes are visible to end-users: yes

• Searched for relevant documentation and updated as needed: yes
• Breaking change (forces users to change their own code or config): no
• Suggested release notes appear below: yes

Test plan

• New test cases added

Git attributes for disabling format checks now support both unset values and true. This allows for more flexible configuration when excluding files from formatting.

[alexeagle]

Thanks!