Like this:
$ git clone --branch dotnet-mono-eclass-lat --depth 1 https://anongit.gentoo.org/git/repo/gentoo.git
From man git-clone
:
--branch <name>, -b <name>
Instead of pointing the newly created HEAD to the branch pointed to by the cloned repository’s HEAD,
point to <name> branch instead. In a non-bare repository, this is the branch that will be checked out.
--depth <depth>
Create a shallow clone with a history truncated to the specified number of revisions. A shallow
repository has a number of limitations (you cannot clone or fetch from it, nor push from nor into
it), but is adequate if you are only interested in the recent history of a large project with a long
history, and would want to send in fixes as patches.