CustomTarget: Use mesonlib.File objects as-is in the command to be run

This allows us to output either the relative or absolute path as
requested. Fixes usage of configure_file inside CustomTarget commands
with the VS backends.

mesonbuild/backend/backends.py

@@ -500,6 +500,10 @@ class Backend():
                 # it assumes that it is in path, so always give it a full path.
                 tmp = i.get_filename()[0]
                 i = os.path.join(self.get_target_dir(i), tmp)
+            elif isinstance(i, mesonlib.File):
+                i = os.path.join(i.subdir, i.fname)
+                if absolute_paths:
+                    i = os.path.join(self.environment.get_build_dir(), i)
             # FIXME: str types are blindly added and ignore the 'absolute_paths' argument
             elif not isinstance(i, str):
                 err_msg = 'Argument {0} is of unknown type {1}'

mesonbuild/build.py

@@ -989,7 +989,7 @@ class CustomTarget:
         for i, c in enumerate(cmd):
             if hasattr(c, 'held_object'):
                 c = c.held_object
-            if isinstance(c, str):
+            if isinstance(c, str) or isinstance(c, File):
                 final_cmd.append(c)
             elif isinstance(c, dependencies.ExternalProgram):
                 if not c.found():
@@ -1005,8 +1005,6 @@ class CustomTarget:
                     if not isinstance(s, str):
                         raise InvalidArguments('Array as argument %d contains a non-string.' % i)
                     final_cmd.append(s)
-            elif isinstance(c, File):
-                final_cmd.append(os.path.join(c.subdir, c.fname))
             else:
                 raise InvalidArguments('Argument %s in "command" is invalid.' % i)
         self.command = final_cmd