GitHub relative link in Markdown file

Question

Is there a way to create a URL anchor, <a>, link from within a Markdown file, to another file within the same repository and branch (aka a link relative to the current branch)?

For example, in the master branch I have a README.md file, which I would like do something like:

# My Project
is really really cool. My Project has a subdir named myLib, see below.

## myLib documentation
see documentation [here](myLib/README.md)

This would allow me to link from one .md to another within the same branch and not have to worry about which branch I'm in (avoid having to do an absolute URL that includes the github.com branch name).

Here is a working example of what I mean:

1. GOTO http://github.com/rynop/testRel, link does not work.
2. GOTO http://github.com/rynop/testRel/blob/master/README.md, link works.

This is expected because at this point the starting URL is in the branch. Now how do I get it to pick up the current branch in the README.md at the root of the repository?

Update: I opened an issue against GitHub for this feature request.

Answer 1

Update 30th, January 2013, 16 months later:

GitHub Blog Post Relative links in markup files:

Starting today, GitHub supports relative links in markup files.
Now you can link directly between different documentation files, whether you view the documentation on GitHub itself, or locally, using a different markup renderer.

You want examples of link definitions and how they work? Here's some Markdown for you.
Instead of an absolute link:

[a link](https://github.com/user/repo/blob/branch/other_file.md)

…you can use a relative link:

[a relative link](other_file.md)
[a relative link](path%20with%20spaces/other_file.md)

and we'll make sure it gets linked to user/repo/blob/branch/other_file.md.

If you were using a workaround like [a workaround link](repo/blob/master/other_file.md), you'll have to update your documentation to use the new syntax.

This also means your documentation can now easily stand on its own, without always pointing to GitHub.

Marcono1234 adds in the comments

Also useful: When the link starts with a /, it is relative to the root of the repository (regardless of whether the markdown file is nested in subdirectories)

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Update December 20th, 2011:

The GitHub markup issue 84 is currently closed by technoweenie, with the comment:

We tried adding a <base> tag for this, but it causes problems with other relative links on the site.

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October 12th, 2011:

If you look at the raw source of the README.md of Markdown itself(!), relative paths don't seem to be supported.
You will find references like:

[r2h]: http://github.com/github/markup/tree/master/lib/github/commands/rest2html
[r2hc]: http://github.com/github/markup/tree/master/lib/github/markups.rb#L13

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As noted in InvisibleWolf's answer, if the target link is a directory and it has space, then you need to use %20 for each space.