Fix ArgumentOutOfRangeException when removing files from game root

[HebaruSan]

Problem

If you install a module that has "install_to": "GameRoot", you'll get an exception when you try to upgrade or remove it:

$ ckan remove KSPLogiRGB-battlemoose
About to remove:

 * KSP Logitech RGB Control v1.0.3

Continue? [Y/n] 
Removing KSPLogiRGB-battlemoose...

Unhandled Exception:
System.ArgumentOutOfRangeException: Index and count must refer to a location within the string.
Parameter name: count
  at System.String.Remove (System.Int32 startIndex, System.Int32 count) [0x0004c] in <9689214c1e4645be91df75196bac3cbb>:0 
  at CKAN.KSPPathUtils.ToRelative (System.String path, System.String root) [0x00064] in <cf1172f4644841da8b9b83c88c51f181>:0 
  at CKAN.ModuleInstaller+<>c__DisplayClass34_0.<AddParentDirectories>b__1 (System.String dir) [0x0004f] in <cf1172f4644841da8b9b83c88c51f181>:0 
  at System.Linq.Enumerable+SelectManySingleSelectorIterator`2[TSource,TResult].MoveNext () [0x00051] in <8107f84c62ea4c799b6ff1411a22ee10>:0 
  at System.Linq.Enumerable+WhereEnumerableIterator`1[TSource].MoveNext () [0x0004e] in <8107f84c62ea4c799b6ff1411a22ee10>:0 
  at System.Collections.Generic.HashSet`1[T].UnionWith (System.Collections.Generic.IEnumerable`1[T] other) [0x00026] in <8107f84c62ea4c799b6ff1411a22ee10>:0 
  at System.Collections.Generic.HashSet`1[T]..ctor (System.Collections.Generic.IEnumerable`1[T] collection, System.Collections.Generic.IEqualityComparer`1[T] comparer) [0x0004b] in <8107f84c62ea4c799b6ff1411a22ee10>:0 
  at System.Collections.Generic.HashSet`1[T]..ctor (System.Collections.Generic.IEnumerable`1[T] collection) [0x00007] in <8107f84c62ea4c799b6ff1411a22ee10>:0 
  at CKAN.Extensions.ToHashSet[T] (System.Collections.Generic.IEnumerable`1[T] source) [0x00000] in <cf1172f4644841da8b9b83c88c51f181>:0 
  at CKAN.ModuleInstaller.AddParentDirectories (System.Collections.Generic.HashSet`1[T] directories) [0x00091] in <cf1172f4644841da8b9b83c88c51f181>:0 
  at CKAN.ModuleInstaller.Uninstall (System.String modName) [0x00105] in <cf1172f4644841da8b9b83c88c51f181>:0 
  at CKAN.ModuleInstaller.UninstallList (System.Collections.Generic.IEnumerable`1[T] mods) [0x00131] in <cf1172f4644841da8b9b83c88c51f181>:0 
  at CKAN.CmdLine.Remove.RunCommand (CKAN.KSP ksp, System.Object raw_options) [0x0016d] in <cf1172f4644841da8b9b83c88c51f181>:0 
  at CKAN.CmdLine.MainClass.RunSimpleAction (CKAN.CmdLine.Options cmdline, CKAN.CmdLine.CommonOptions options, System.String[] args, CKAN.IUser user, CKAN.KSPManager manager) [0x003d8] in <cf1172f4644841da8b9b83c88c51f181>:0 
  at CKAN.CmdLine.MainClass.Execute (CKAN.KSPManager manager, CKAN.CmdLine.CommonOptions opts, System.String[] args) [0x0015b] in <cf1172f4644841da8b9b83c88c51f181>:0 
  at CKAN.CmdLine.MainClass.Main (System.String[] args) [0x000a8] in <cf1172f4644841da8b9b83c88c51f181>:0 
[ERROR] FATAL UNHANDLED EXCEPTION: System.ArgumentOutOfRangeException: Index and count must refer to a location within the string.

Cause

When a file is in GameRoot, path and root are the same on this line:

CKAN/Core/KSPPathUtils.cs

Lines 246 to 249 in 86bf144

	// The +1 here is because root will never have
	// a trailing slash.
	return path.Remove(0, root.Length + 1);
	}

So the second parameter of the Remove call ends up being longer than path, which causes the exception.

Changes

Now we compare the lengths of the strings before calling Remove. If they're the same, we just return the empty string.

$ ckan remove KSPLogiRGB-battlemoose
About to remove:

 * KSP Logitech RGB Control v1.0.3

Continue? [Y/n] 
Removing KSPLogiRGB-battlemoose...
Done!

Fixes #2330.