feat: --filter-file-path to support relative paths

[leonardochaia]

Adds support for --filter-file-path to support relative paths.

When --filter-file-path is relative, it is always relative to the current working directory, even if a --repository-path is provided

This table covers al supported cases:

Repository Path	Filter File Path	Expected Repository Path	Expected Filter File Path
/home/lchaia/dotnet-affected	""	/home/lchaia/dotnet-affected	null
""	/home/lchaia/dotnet-affected/Affected.sln	/home/lchaia/dotnet-affected	/home/lchaia/dotnet-affected/Affected.sln
/home/lchaia/dotnet-affected	/home/lchaia/dotnet-affected/subdirectory/other/Affected.sln	/home/lchaia/dotnet-affected	/home/lchaia/dotnet-affected/subdirectory/other/Affected.sln
""	""	Environment.CurrentDirectory	null
""	Affected.sln	Environment.CurrentDirectory	Path.Join(Environment.CurrentDirectory, "Affected.sln")
/home/lchaia/dotnet-affected	Affected.sln	/home/lchaia/dotnet-affected	Path.Join(Environment.CurrentDirectory, "Affected.sln")

Fixes #105