GitHub relative link in Markdown file

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)

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.

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

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.

For example, you have a repo like the following:

project/
    text.md
    subpro/
       subtext.md
       subsubpro/
           subsubtext.md
       subsubpro2/
           subsubtext2.md

The relative link to subtext.md in text.md might look like this:

[this subtext](subpro/subtext.md)

The relative link to subsubtext.md in text.md might look like this:

[this subsubtext](subpro/subsubpro/subsubtext.md)

The relative link to subtext.md in subsubtext.md might look like this:

[this subtext](../subtext.md)

The relative link to subsubtext2.md in subsubtext.md might look like this:

[this subsubtext2](../subsubpro2/subsubtext2.md)

The relative link to text.md in subsubtext.md might look like this:

[this text](../../text.md)

GitHub could make this a lot better with minimal work. Here is a work-around.

I think you want something more like

[Your Title](your-project-name/tree/master/your-subfolder)

or to point to the README itself

[README](your-project-name/blob/master/your-subfolder/README.md)

As of January 31, 2013 Github markdown supports relative links to files.

[a relative link](markdown_file.md)

However, there are a few deficiencies that have been discussed in this comment thread.

As an alternative, you can use Gitdown to construct full URLs to the repository and even make them branch aware, e.g.

{"gitdown": "gitinfo", "name": "url"} // https://github.com/gajus/gitdown
{"gitdown": "gitinfo", "name": "branch"} // master

Gitdown is a GitHub markdown preprocessor. It streamlines common tasks associated with maintaining a documentation page for a GitHub repository, e.g. generating table of contents, including variables, generating URLs and getting information about the repository itself at the time of processing the input. Gitdown seamlessly integrates with your building scripts.

I am the author of the Gitdown library.