VSCode opens the same file in multiple editors if the names use different cases

[wm1]

• VSCode Version: 1.8.1
• OS Version: Windows 10

Steps to Reproduce:

1. Create a file named LICENSE.txt

2. Run "Code --reuse-window LICENSE.txt"

3. Run "Code --reuse-window license.txt"

4. Observe that VSCode opens 2 editor windows.

5. Expect: VSCode detects the two file names are actually the same file, and only opens one file instead.

   

This is especially annoying when dealing with MSVC project, when click the build error message and a new window is opened.

1. Create a C++ project. The file name contains mixed upper and lower case. Open the file d:\git\Test\Main.cpp in editor
2. Create a task.json and use msbuild to build the project
3. Build. Assume there is an syntax error like the following. Notice that cl.exe converts filename to lower case
   d:\git\test\main.cpp(10): error C2144: syntax error: 'int' should be preceded by ';' [d:\git\Test\Test.vcxproj]
4. Click the status "Problem" icon. Select the error message from the panel. The main.cpp is opened in editor, right next to Main.cpp

[bpasero]

Let's merge into #12448