Rename trunk branch to master

[drizt]

git svn clone creates trunk branch. But in git default branch name is master so it should be renamed. It need for comfortable work with git. For example I can do

git checkout -b my_test
....
git commit -m 'My test commit'
# Next I want to return to default branch as I do it always.
git checkout master 

Atm the last command will fail and it will be unexpected behaviour.