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

This commit in order to prevent fail when only revision is specified
and remote depository do not have 'master' branch.

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', GIT_DEFAULT_BRANCH_NAME)
+    self.branch = options.get('branch')
     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 != GIT_DEFAULT_BRANCH_NAME:
+    if self.revision and self.branch:
       # 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.')