fix #1227: relative path in pages that use permalink

[shigma]

Summary

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Build-related changes
• Other, please describe:

If changing the UI of default theme, please provide the before/after screenshot:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

---

Impact: This PR changes the behaviour of relative urls in markdown files that use permalink

docs
├── with-permalink.md
└── target-page.md

• One page uses permalink, in which there is a link to the other page.
• The other page does not use permalink.

Case 1

// with-permalink.md
---
permalink: '/foo/bar'
---

[To target page](./target-page.md)

before: 404
after: link to target-page.md

Case 2

// with-permalink.md
---
permalink: '/foo/bar'
---

[To target page](./../../target-page.md)

before: link to target-page.md
after: 404

Migration

• change urls in pages that use permalink in favor of the file system rather than the permalink
• use absolute urls (which was recommended by [Bug report] page links failed in pages that use permalink #1227 (comment))

---

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)

You have tested in the following browsers: (Providing a detailed version will be better.)

• Chrome 71.0.3578.98
• Firefox
• Safari
• Edge
• IE

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature
• Related documents have been updated
• Related tests have been updated

To avoid wasting your time, it's best to open a feature request issue first and wait for approval before working on it.

Other information:

The cases was provided in #1227 .

[shigma]

Some additional comments

1. Added some markdown-it plugin environment api:
   • env.relPath: the relative path of the file in source directory
   • env.frontmatter: the frontmatter in the markdown file
2. Fixed a typo.

[ulivz]

cc @meteorlxy

[meteorlxy]

Great work @shigma ! This implementation is much simpler than I thought.

I've tested it on Windows 10 and it works well.

[meteorlxy]

Besides the cases we listed, there are some other cases:

• From normal link page to permalink page
• From permalink page to permalink page
• From permalink page to normal link page (the listed case)

I've roughly tested all those cases and they work well, as we have already added redirect routes from regularPath to its permalink.

👍