proposal: make logging cleaner by printing %o et al relative to the variant dir

[uohmak4fvpqe]

Consider :

: some_input_files |> ^ %o^ the_command |> output_name

I like to display the output file (e.g. "file.o"), not a command id plus input files as is traditional (stuff like "CC file.c") The problem is, when using variants the print is redundant and too verbose, as in:

[0.007s] [build-release-native-gnu] pro/src: ../../../build-release-native-gnu/pro/src/../file.o

when I would like

[0.007s] [build-release-native-gnu] pro/src: ../file.o

I.e, I would like the logging to display %o in the shortest form relative to the prefixed directory in [ ... ] ... :, not relative to the source working directory.