When a issue is moved from one project to another, all associated
Markdown text is rewritten in the context of the new project. If the
note contained a link to a commit URL, `CommitRewriter#rewrite` would
fail because `Commit#link_reference_pattern` would match `nil` `commit`
values in the HTML generated from the Markdown. These `nil` values were
passed along to `Project#commits_by` because `Commit#reference_valid?`
was always returning `true`.

To prevent this issue from happening, we tighten up the check for
`Commit#reference_valid?` to look for valid SHA values.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/66666

This reverts commit 96bef54154e669f9a3e92c3a4bc76c0be3a52e48.

disable markdown in comments when referencing disabled features

fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/23548

This MR prevents the following references when tool is disabled:

- issues
- snippets
- commits - when repo is disabled
- commit range - when repo is disabled
- milestones

This MR does not prevent references to repository files, since they are just markdown links and don't leak
information.

See merge request !2011
Signed-off-by: NRémy Coutable <remy@rymai.me>

This reverts commit 3e991230.

# Conflicts:
#	app/models/project.rb

Also removes the note from the development/testing.md guide

Commit 43d8bdb4 introduced additional
checks for permissions to read issue in references extractor.