Slightly improved "diff-pr" output, especially around build-order

This should help avoid "empty cat" output we saw recently when something failed to fetch but was otherwise valid.

The current implementation is trying to do two things with one output:

1. normalized `bashbrew cat` diff
2. visualize `--build-order` changes

This change splits those into two separate things because combining them ends up making the diff harder to read than it should be ("changed tags + changed commit" leading to a full reordering of the diff because build-order also changed).

_bashbrew-cat-sorted.sh

@@ -5,8 +5,7 @@ set -Eeuo pipefail
 
 images="$(
 	bashbrew list --repos --uniq "$@" \
-		| sort -uV \
-		| xargs -r bashbrew list --repos --uniq --build-order
+		| sort -uV
 )"
 set -- $images
 
@@ -34,7 +33,6 @@ for img; do
 
 	bashbrew list --uniq "$img" \
 		| sort -V \
-		| xargs -r bashbrew list --uniq --build-order \
 		| xargs -r bashbrew cat --format '
 			{{- range $e := .TagEntries -}}
 				{{- printf "\n%s\n" ($e.ClearDefaults $.Manifest.Global) -}}

diff-pr.sh

@@ -249,6 +249,35 @@ copy-tar() {
 	done
 }
 
+# a "bashbrew cat" template that gives us the last / "least specific" tags for the arguments
+# (in other words, this is "bashbrew list --uniq" but last instead of first)
+templateLastTags='
+	{{- range .TagEntries -}}
+		{{- $.RepoName -}}
+		{{- ":" -}}
+		{{- .Tags | last -}}
+		{{- "\n" -}}
+	{{- end -}}
+'
+
+_metadata-files() {
+	bashbrew list "$@" 2>>temp/_bashbrew.err | sort -uV > temp/_bashbrew-list || :
+
+	"$diffDir/_bashbrew-cat-sorted.sh" "$@" 2>>temp/_bashbrew.err > temp/_bashbrew-cat || :
+
+	bashbrew list --uniq "$@" \
+		| sort -V \
+		| xargs -r bashbrew list --uniq --build-order 2>>temp/_bashbrew.err \
+		| xargs -r bashbrew cat --format "$templateLastTags" 2>>temp/_bashbrew.err \
+		> temp/_bashbrew-list-build-order || :
+
+	script="$(bashbrew cat --format "$template" "$@")"
+	mkdir tar
+	( eval "$script" | tar -xiC tar )
+	copy-tar tar temp
+	rm -rf tar
+}
+
 mkdir temp
 git -C temp init --quiet
 git -C temp config user.name 'Bogus'
@@ -257,15 +286,7 @@ git -C temp config user.email 'bogus@bogus'
 # handle "new-image" PRs gracefully
 for img; do touch "$BASHBREW_LIBRARY/$img"; [ -s "$BASHBREW_LIBRARY/$img" ] || echo 'Maintainers: New Image! :D (@docker-library-bot)' > "$BASHBREW_LIBRARY/$img"; done
 
-bashbrew list "$@" 2>>temp/_bashbrew.err | sort -uV > temp/_bashbrew-list || :
-"$diffDir/_bashbrew-cat-sorted.sh" "$@" 2>>temp/_bashbrew.err > temp/_bashbrew-cat || :
-for image; do
-	script="$(bashbrew cat --format "$template" "$image")"
-	mkdir tar
-	( eval "$script" | tar -xiC tar )
-	copy-tar tar temp
-	rm -rf tar
-done
+_metadata-files "$@"
 git -C temp add . || :
 git -C temp commit --quiet --allow-empty -m 'initial' || :
 
@@ -276,13 +297,8 @@ git -C oi checkout --quiet pull
 for img; do touch "$BASHBREW_LIBRARY/$img"; [ -s "$BASHBREW_LIBRARY/$img" ] || echo 'Maintainers: Deleted Image D: (@docker-library-bot)' > "$BASHBREW_LIBRARY/$img"; done
 
 git -C temp rm --quiet -rf . || :
-bashbrew list "$@" 2>>temp/_bashbrew.err | sort -uV > temp/_bashbrew-list || :
-"$diffDir/_bashbrew-cat-sorted.sh" "$@" 2>>temp/_bashbrew.err > temp/_bashbrew-cat || :
-script="$(bashbrew cat --format "$template" "$@")"
-mkdir tar
-( eval "$script" | tar -xiC tar )
-copy-tar tar temp
-rm -rf tar
+
+_metadata-files "$@"
 git -C temp add .
 
 git -C temp diff \