Cosmetic tweak to error message for incdir() with an absolute path

Cosmetic tweak to the error message for incdir() with an absolute path.
Don't split the message in the middle of a sentence at a point which may
or may not correspond to the terminal width.

mesonbuild/interpreter.py

@@ -4220,8 +4220,9 @@ This will become a hard error in the future.''' % kwargs['input'], location=self
 
         for a in incdir_strings:
             if a.startswith(src_root):
-                raise InvalidArguments('''Tried to form an absolute path to a source dir. You should not do that but use
+                raise InvalidArguments('Tried to form an absolute path to a source dir. '
-relative paths instead.
+                                       'You should not do that but use relative paths instead.'
+                                       '''
 
 To get include path to any directory relative to the current dir do