Lint is inconsistent on how source paths are reported

[thomasvl]

1. Checkout https://github.com/apple/swift-protobuf @ c9fbc332ed293c64ff70af8d82367ca530dce11c
2. Run

   swift format lint \
       Sources/SwiftProtobuf/Message.swift \
       Tests/SwiftProtobufTests/Test_Extensions.swift

You get:

Sources/SwiftProtobuf/Message.swift:46:16: warning: [DontRepeatTypeInStaticProperties] remove the suffix 'Message' from the name of the variable 'protoMessageName'
/Users/thomasvl/src/github/swift-protobuf/Tests/SwiftProtobufTests/Test_Extensions.swift:97:68: warning: [EndOfLineComment] move end-of-line comment that exceeds the line length

The order of the two arguments doesn't matters, the path to Message.swift is reported as the shortened relative path and Test_Extensions.swift is always reported as a full path instead.

[ahoppen]

Synced to Apple’s issue tracker as rdar://136639896