Fix root path on Windows

User may use lowercase drive letter when entering gclient workspace.
In such case, path should be corrected to real path value.

R=wfh@chromium.org

Fixed: 401141847
Change-Id: I0b08985ec5c911a315922858d248ff53a10ed682
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6336450
Auto-Submit: Josip Sokcevic <sokcevic@chromium.org>
Reviewed-by: Yiwei Zhang <yiwzhang@google.com>
Commit-Queue: Yiwei Zhang <yiwzhang@google.com>

git_cl.py

@@ -862,7 +862,8 @@ class Settings(object):
 
     def GetRoot(self):
         if self.root is None:
-            self.root = os.path.abspath(self.GetRelativeRoot())
+            self.root = os.path.realpath(os.path.abspath(
+                self.GetRelativeRoot()))
         return self.root
 
     def GetTreeStatusUrl(self, error_ok=False):