Select PR template by <source> and <target> branch included in template's filename

[mbronk]

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Even though multiple PR templates are supported, their selection seems possible only via URL manipulation (prompting to create a separate UI on top of GH in order to get a template chooser experience).

Since the "big" overhaul of the Pull Request Template experience seems heavy and not done for years¹, I propose a simpler move in this direction:

🌟 Heuristically match PR template file by <source> and <target> branch name embedded in the template name

---

💡 Core idea

Aside the catch-all default template .github/PULL_REQUEST_TEMPLATE.md have a file naming convention² of the .github\PULL_REQUEST_TEMPLATE\*.md files, following:

• {baseRefName}^{compareRefName}.md ^{(or similar3)}
  with the refnames matched as substrings (or using globbing patterns, prefixes, etc.).

For example, given a PR template: .github\PULL_REQUEST_TEMPLATE\main^feature.md, every PR targeting main (if the source matches *feature*) would get this PR template, but a PR main <- personal-experiment would not (and fall back to the default).
Similarly:

• .github\PULL_REQUEST_TEMPLATE\main^.md intercepts anything targeting *main*, and
• .github\PULL_REQUEST_TEMPLATE\^private.md intercepts anything sourced from *private*

Rationale

It's not a full-blown solution to all the templating needs, but caters to one of the more typical problems: i.e. certain branches (source, target or both) requiring different rules⁴. Such naive template choosing should have the benefit of being relatively cheap to implement (hopefully by merely adding a stage to the template lookup strategy) and not collide with existing naming schemes in use (who uses ^ in their filenames? 😃 ).

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Scaled-down version

Only focus on the target branch name and use its full name verbatim. I.e. assuming we have

.
├── .github
│   ├── PULL_REQUEST_TEMPLATE
│   │   └── main.md  #Template1
│   └── PULL_REQUEST_TEMPLATE.md   #Template2 (default)

Any PR targeting main will get Template1 and everything else will fall back to Template2

---

Footnotes

1. There are already a few great ideas on how to improve the overall UX with not much progress since a few years already 😞 . Just to name a few:
   - Pull Request Templates improvements: support `.github-private` and UI for picking template #42499
   - Add Pull request Template chooser similar to Issue Template chooser #4620
   - Pull Request Templates Scoped to Directory #8229 ↩

2. Better yet, metadata inside the .md files. ↩

3. I.e. BASE[{baseRefName}]___COMPARE[{compareRefName}]. Ideally separators could be chosen in such a way so that they're valid cross-OS file names, but (not valid for a refname)[https://git-scm.com/docs/git-check-ref-format]. ^ seems a good candidate ❔ ↩

4. For example: private/feature branches being OK with a short description but a stable/hotfix branch requiring to add additional external IDs etc. ↩

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