gitclone: empty parameter equals no parameter.

It allows to easily extend an existing section

README.rst

@@ -188,6 +188,39 @@ When updating, it will do a "git fetch; git reset revision"::
   Updating git-clone.
   Fetching origin
   HEAD is now at ...
+Empty revision/branch
+---------------------
+
+Specifying an empty revision or an empty branch will make buildout
+ignore those values as if it was not present at all (allowing to easily
+extend an existing section specifying a branch)::
+
+  >>> cd(sample_buildout)
+  >>> write(sample_buildout, 'buildout.cfg',
+  ... """
+  ... [buildout]
+  ... parts = git-clone
+  ...
+  ... [git-clone-with-branch]
+  ... recipe = slapos.recipe.build:gitclone
+  ... repository = http://git.erp5.org/repos/slapos.recipe.build.git
+  ... revision = 2566127
+  ...
+  ... [git-clone]
+  ... <= git-clone-with-branch
+  ... revision =
+  ... branch = master
+  ... """)
+
+  >>> print system(buildout)
+  Uninstalling git-clone.
+  Running uninstall recipe.
+  Installing git-clone.
+  Cloning into '/sample-buildout/parts/git-clone'...
+
+  >>> cd(sample_buildout, 'parts', 'git-clone')
+  >>> print system('git branch')
+  * master
 
 Specific git binary
 -------------------

slapos/recipe/gitclone.py

@@ -126,12 +126,16 @@ class Recipe(object):
   def __init__(self, buildout, name, options):
     options.setdefault('location',
         os.path.join(buildout['buildout']['parts-directory'], name))
-    self.repository = options.get('repository')
-    self.branch = options.get('branch')
-    self.revision = options.get('revision')
-    self.git_command = options.get('git-executable', 'git')
     self.name = name
-    self.location = options.get('location')
+    for option in ('branch', 'revision', 'location', 'repository'):
+      value = options.get(option, '').strip()
+      if value == '':
+        setattr(self, option, None)
+      else:
+        setattr(self, option, value)
+    self.git_command = options.get('git-executable', '')
+    if self.git_command == '':
+      self.git_command = 'git'
     # Set boolean values
     for key in ('develop', 'use-cache', 'ignore-ssl-certificate'):
       setattr(self, key.replace('-', '_'), options.get(key) in TRUE_VALUES)