Paths starting with `./` in DefaultItemExcludes are not properly handled

This is probably a flaw in the implementation of [FileMatcher](/microsoft/msbuild/blob/vs16.5/src/Shared/FileMatcher.cs).

### Steps to reproduce

See [here](https://github.com/lanfeust69/TestDefaultItemExcludes)

```
msbuild /bl /p:WithDot=True
```
### Expected behavior

`None` items should only contain the `.git*` files and `ReadMe.md`

### Actual behavior

`None` items also include the `bin` and `Data` contents.

### Environment data
`msbuild /version` output:
`16.5.0.12403`

OS info:
both Windows 10 and Ubuntu 19.10

Start with src/Shared/FileMatcher.cs and reproduce the issue using the linked example with `msbuild /bl /p:WithDot=True`. Compare the `None` items produced for paths beginning with `./` against the expected `.git*` files and `ReadMe.md`; done means `bin` and `Data` contents are excluded on both reported environments.

Written by the indexing model from the issue text.