Capture diffs of dist tarballs vs auto-generated source archives from GitHub

[bwarden]

Highlight files that are different or only present in the release-manager-provided distribution tarballs on GitHub repositories vs. the tarball that GitHub automatically generates for the corresponding tag.

[bwarden]

Using diff -u -r <main_src> <autogenerated_src>, so we currently don't emit contents of files that exist only in one or the other.

When run against a couple of sample packages, the results look like:

libunibreak

diff --git a/archive.diff b/archive.diff
new file mode 100644
index 000000000000..3c12e25b0faa
--- /dev/null
+++ b/archive.diff
@@ -0,0 +1,20 @@
+Only in libunibreak-5.1: aclocal.m4
+Only in autogenerated/libunibreak-libunibreak_5_1: autogen.sh
+Only in autogenerated/libunibreak-libunibreak_5_1: bootstrap
+Only in autogenerated/libunibreak-libunibreak_5_1: .clang-format
+Only in libunibreak-5.1: compile
+Only in libunibreak-5.1: config.guess
+Only in libunibreak-5.1: config.h.in
+Only in libunibreak-5.1: config.sub
+Only in libunibreak-5.1: configure
+Only in libunibreak-5.1: depcomp
+Only in libunibreak-5.1: doc
+Only in autogenerated/libunibreak-libunibreak_5_1: .gitignore
+Only in libunibreak-5.1: install-sh
+Only in libunibreak-5.1: ltmain.sh
+Only in libunibreak-5.1: m4
+Only in autogenerated/libunibreak-libunibreak_5_1: .mailmap
+Only in libunibreak-5.1: Makefile.in
+Only in libunibreak-5.1: missing
+Only in autogenerated/libunibreak-libunibreak_5_1: purge
+Only in libunibreak-5.1/src: Makefile.in

zstd (no difference between the official release tarball and the tag-based auto-generated tarball)

diff --git a/archive.diff b/archive.diff
new file mode 100644
index 000000000000..e69de29bb2d1

[bwarden]

Need to figure out how to handle the prefix so we don't get a big diff just from the source version number changing.

[bwarden]

Don't feel rushed to merge this; I'm looking for any input on the approach.

[bryteise]

When you think this is about ready, would you mind squashing it?