Add option to avoid GitHub backreferencing the PR

Closes #26

niv-update-action changelog in a GitHub PR contains references (in the form
of `owner/repo#PR_number`), which GitHub nicely turns into links.
However, what GitHub also does is it creates a backreference in those
PRs back to the PR created by niv-update-action. This pollutes the
upstream projects. Thus, this change adds an option to use a redirector
service in order to still have links, but avoid having backreferences.

README.adoc

@@ -116,6 +116,12 @@ jobs:
   changelog. Defaults to empty.
 * `title_prefix`: (Optional) The text that will be put in front of the
   generated commit title. Defaults to empty.
+* `github_changelog_no_backreferences`: (Optional, a boolean) If `true`, the
+  changelog will transform all issue links to links via a redirector
+  (DuckDuckGo), to prevent GitHub from backreferencing the created PR in these
+  issues. For more details, see
+  https://github.com/knl/niv-updater-action/issues/26[Issue #26]. Defaults to
+  `false`.
 
 == Secrets
 

action.yml

@@ -53,6 +53,10 @@ inputs:
     description: 'The text that will be put in front of the generated commit title. Defaults to empty.'
     required: false
     default: ''
+  github_changelog_no_backreferences:
+    description: 'If `true`, the changelog will transform all issue links to links via a redirector, to prevent GitHub from backreferencing the created PR in these issues. Defaults to `false`.'
+    required: false
+    default: false
 
 branding:
   # maybe 'refresh-cw'

niv-updater

@@ -107,6 +107,36 @@ applyLabels() {
     fi
 }
 
+# This function will turn any #123 into owner/repo#123.
+# This is needed, since the changelogs from different repositories will all have issue links as #123,
+# which, when printed as a PR description will create a link to the _current_ repository (one with
+# nix/sources.json), not one where the changelog originated from. The owner/repo#123 format will
+# ensure proper linking.
+formatIssueLinksPlain() {
+    dep_owner="$1"
+    dep_repo="$2"
+    sed "s~\(#[0-9]\+\)~$dep_owner/$dep_repo\1~g"
+}
+
+# Like formatIssuesLinkPlain, this function turns any #123 a direct reference to the original repository.
+# Unlike formatIssuesLink, it will use a redirection service (DuckDuckGo), so that referenced PRs/Issues
+# do not contain back references.
+# For more details, see https://github.com/knl/niv-updater-action/issues/26
+formatIssueLinksNoBackreferences() {
+    dep_owner="$1"
+    dep_repo="$2"
+    sed "s~#\([0-9]\+\)~[$dep_owner/$dep_repo#\1](https://duckduckgo.com/l/?kh=-1&uddg=https://github.com/$dep_owner/$dep_repo/issues/\1)~g"
+}
+
+# A dispatcher for formatIssueLinksPlain and formatIssueLinksNoBackreferences, based on config.
+formatIssueLinks() {
+    if [[ $INPUT_GITHUB_CHANGELOG_NO_BACKREFERENCES == "true" ]]; then
+        formatIssueLinksNoBackreferences "$@"
+    else
+        formatIssueLinksPlain "$@"
+    fi
+}
+
 createPullRequestsOnUpdate() {
     echo "Checking for updates"
     if [[ -z $INPUT_PULL_REQUEST_BASE ]]; then
@@ -271,7 +301,10 @@ createPullRequestsOnUpdate() {
                 printf "## Changelog for %s:\n" "$dep"
                 printf "Branch: %s\n" "$niv_branch"
                 printf "Commits: [$dep_owner/$dep_repo@%.8s...%.8s](https://github.com/$dep_owner/$dep_repo/compare/${revision}...${new_revision})\n\n" "$revision" "$new_revision"
-                { hub api "/repos/$dep_owner/$dep_repo/compare/${revision}...${new_revision}" || true; } | jq -r '.commits[] '"$merges_filter"' | "* [`\(.sha[0:8])`](\(.html_url)) \(.commit.message | split("\n") | first)"' | sed "s~\(#[0-9]\+\)~$dep_owner/$dep_repo\1~g"
+                {
+                    hub api "/repos/$dep_owner/$dep_repo/compare/${revision}...${new_revision}" || true
+                } | jq -r '.commits[] '"$merges_filter"' | "* [`\(.sha[0:8])`](\(.html_url)) \(.commit.message | split("\n") | first)"' \
+                    | formatIssueLinks "$dep_owner" "$dep_repo"
             } >>"$message"
         fi