I have a Mercurial repository containing a handful of related projects. I want to branch just one of these projects to work on it elsewhere.
Is cloning just part of a repository possible, and is that the right way to achieve this?
What you want is a narrow or partial clone, but this is unfortunately not yet supported.
If you already have a big repository and you realize that it would make sense to split it into several smaller repositories, then you can use the convert extension to do a Mercurial to Mercurial conversion. Note that this creates a new repository foo and you cannot push/pull between your-big-repo and foo.
The convert extension is not enabled by default so add the following to your repo's hgrc file or your mercurial.ini file:
[extensions]
hgext.convert=
Then create a map.txt file with
include "libs/foo"
rename "libs/foo" .
(note you can use forward slashes even on Windows) and run
$ hg convert --filemap map.txt your-big-repo foo
That will make foo a repository with the full history of the libs/foo folder from your-big-repo.
If you want to delete all evidence of foo from your-big-repo you can make another conversion where you use exclude libs/foo to get rid of the directory.
When you have several repositories like that and you want to use them as a whole, then you should look at subrepositories. This feature lets you include other repositories in a checkout — similarly to how svn:externals work. Please follow the recommendations on that wiki page.