Revert "gitclone: do not force to use 'master' branch when not specified."

This reverts commit 94446fa8.

This commit is blocking to set revision with branch equal master. It
means that the code below is not allowed.

[target]
revision = 1234
branch = master

slapos/recipe/gitclone.py

@@ -126,7 +126,7 @@ class Recipe(object):
     options.setdefault('location',
         os.path.join(buildout['buildout']['parts-directory'], name))
     self.repository = options.get('repository')
-    self.branch = options.get('branch')
+    self.branch = options.get('branch', GIT_DEFAULT_BRANCH_NAME)
     self.revision = options.get('revision')
     self.git_command = options.get('git-executable', 'git')
     self.name = name
@@ -140,7 +140,7 @@ class Recipe(object):
     # Check if input is correct
     if not self.repository:
       raise UserError('repository parameter is mandatory.')
-    if self.revision and self.branch:
+    if self.revision and self.branch != GIT_DEFAULT_BRANCH_NAME:
       # revision and branch options are incompatible
       raise UserError('revision and branch (other than master) parameters '
           'are set but are incompatible. Please specify only one of them.')