pkgdiff-mg: can't cope with ruby-fakegem unpacked layout

[thesamesam]

pkgdiff-mg doesn't currently work for ruby-fakegem.eclass-based ebuilds because it compares the wrong paths, and all differences show up as new files.

(I don't know why this layout exists at the point of src_unpack or if that's optimal, though.)

For example, take d24600a71dade13abf890f10ae29a2d39302b68c in gentoo.git, and run pkgdiff-mg pg-1.5.{5,6}.ebuild.

We get a bunch of results like:

diff '--color=always' '--exclude=.git' --strip-trailing-cr -X - -dupNr /tmp/mgorny-dev-scripts/portage/dev-ruby/pg-1.5.5/work/all/ruby-pg-1.5.6/.travis.yml /tmp/mgorny-dev-scripts/portage/dev-ruby/pg-1.5.6/work/all/ruby-pg-1.5.6/.travis.yml
--- /tmp/mgorny-dev-scripts/portage/dev-ruby/pg-1.5.5/work/all/ruby-pg-1.5.6/.travis.yml        1970-01-01 01:00:00.000000000 +0100
+++ /tmp/mgorny-dev-scripts/portage/dev-ruby/pg-1.5.6/work/all/ruby-pg-1.5.6/.travis.yml        2024-03-01 18:15:55.000000000 +0000
@@ -0,0 +1,49 @@
+sudo: required
+dist: focal
+services:
+  - docker
+language: ruby
+matrix:
+  include:
+    # i386: Intel 32-bit
+    - name: i386
[...]

i.e. every file gets treated as brand new because we're comparing the wrong basepath on the LHS, as we don't realise that there's an extra layer within WORKDIR.

[thesamesam]

cc @graaff

Pretty sure I had a workaround for this ages ago but I've since lost/forgot about it.

[mgorny]

I guess we'd have to use ${S}/${RUBY_S}?