ExcludeByFile add traversal support when full path specified.

[SlowLogicBoy]

Fixes
System.ArgumentException: ".." can be only added at the beginning of the pattern.
when specifying full path with path traversal:
<ExcludeByFile>$(MSBuildThisFileDirectory)../**/*.Designer.cs</ExcludeByFile>

The main problem might be that we can't specify our own root path, it's ether Directory.GetCurrentDirectory() or Path.GetRootPath(pattern).

[codecov]

Codecov Report

Merging #236 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #236   +/-   ##
=======================================
  Coverage   94.91%   94.91%           
=======================================
  Files          15       15           
  Lines        1633     1633           
=======================================
  Hits         1550     1550           
  Misses         83       83

Impacted Files	Coverage Δ
src/coverlet.core/Helpers/InstrumentationHelper.cs	98.46% <100%> (ø)	⬆️

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[tonerdo]

Will need to do some personal testing on this. Any gotchas I should be aware of?

[SlowLogicBoy]

D:\Some\Path\..\..\..\SomeOther\Path: when traversing more then there is path. The only gotcha that bothers me, since I didn't check for that.
Also it might not be a good idea to do Path.GetFullPath(excludeRule) when there is no path traversal (../) since it might add additional overhead (it accesses physical files if it exists).
GetFullPath

EDIT:

D:\Some\Path\..\..\..\SomeOther\Path: when traversing more then there is path. The only gotcha that bothers me, since I didn't check for that.

Looks like it ignores traversal if it's exceeded:
D:\Some\Path\..\..\..\SomeOther\Path becomes D:\SomeOther\Path