.pytool/Plugin/UncrustifyCheck: Update func to return absolute paths

Currently, UncrustifyCheck._get_git_ignored_paths() is documented to
return a list of absolute file paths but it currently returns a list
of relative file paths.

This change updates the function to return a list of absolute file
paths. The result is later compared to the list of absolute file
paths for files to run against Uncrustify.

Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Sean Brogan <sean.brogan@microsoft.com>
Cc: Bret Barkelew <Bret.Barkelew@microsoft.com>
Signed-off-by: Michael Kubacki <michael.kubacki@microsoft.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>

.pytool/Plugin/UncrustifyCheck/UncrustifyCheck.py

@@ -292,7 +292,12 @@ class UncrustifyCheck(ICiBuildPlugin):
                 f"An error occurred reading git ignore settings. This will prevent Uncrustify from running against the expected set of files.")
 
         # Note: This will potentially be a large list, but at least sorted
-        return outstream_buffer.getvalue().strip().splitlines()
+        rel_paths = outstream_buffer.getvalue().strip().splitlines()
+        abs_paths = []
+        for path in rel_paths:
+            abs_paths.append(
+                os.path.normpath(os.path.join(self._abs_workspace_path, path)))
+        return abs_paths
 
     def _get_git_submodule_paths(self) -> List[str]:
         """