get a branch and store it in the directory percona_server_5.5

bzr branch lp:percona-server/5.5 percona_server_5.5

get a branch from a tag

each minor MySQL version is identified by a tag, for example 5.5.27 here major version is 5.5 and that will be the name of the branch and the tag name will have 5.5.27 in it
to find the name of the tag corresponding to a minor release, first go inside the directory corresponding to the branch of the major version, for example in the example above go to the directory percona_server_5.5 and then the following: bzr tags | grep 5.5.27
then what the above returns, use it as follows to fetch the branch corresponding to the tag and store it in the directory percona_server_5.5.27 bzr branch -r tag:Percona-Server-5.5.27-28.0 lp:percona-server/5.5 percona_server_5.5.27

the branch could have been updated since you fetched it locally, so you need to update the branch to get the changes
for that go into the folder of the branch, for example in this case percona_server_5.5 and do the following: bzr pull

bzr log

bzr log -r 3593 -p