The global --topdir-format option governs how a project's root directory is represented in paths output by various queries. "absolute" means as absolute path, "unaltered" means verbatim as specified via --topdir, make:xyz means replaced by the string $(xyz), for later expansion in a makefile variable.
This commit adds another variant: "relative" yields the shortest possible output format of the output path in question relative to --topdir, with "shortest possible" in this context meaning canonicalized and leading "./" stripped.
Signed-off-by: Jan Lindemann <jan@janware.com>
