Match name and result of is_detached() method

Change the is_detached() method to negate the test within the method
call, so that result is True if the tree is in a detached stage,
instead of needing to invert the result from where the program calls
the method.

Change-Id: I30ea105fa354fcd975c134e87f9422d69864de9f

git_upstream/lib/drop.py

@@ -69,7 +69,7 @@ class Drop(LogDedentMixin, GitMixin):
             self._author = author
 
         # test that we can use this git repo
-        if not self.is_detached():
+        if self.is_detached():
             raise DropError("In 'detached HEAD' state")
 
         # To Do: check if it possible and useful.

git_upstream/lib/importupstream.py

@@ -48,7 +48,7 @@ class ImportUpstream(LogDedentMixin, GitMixin):
         super(ImportUpstream, self).__init__(*args, **kwargs)
 
         # test that we can use this git repo
-        if not self.is_detached():
+        if self.is_detached():
             raise ImportUpstreamError("In 'detached HEAD' state")
 
         if self.repo.bare:

git_upstream/lib/supersede.py

@@ -62,7 +62,7 @@ class Supersede(LogDedentMixin, GitMixin):
             raise SupersedeError("Commit should be provided")
 
         # test that we can use this git repo
-        if not self.is_detached():
+        if self.is_detached():
             raise SupersedeError("In 'detached HEAD' state")
 
         # To Do: check if it possible and useful.

git_upstream/lib/utils.py

@@ -53,7 +53,7 @@ class GitMixin(object):
         return self.__git
 
     def is_detached(self):
-        return self.git.symbolic_ref("HEAD", q=True, with_exceptions=False)
+        return not self.git.symbolic_ref("HEAD", q=True, with_exceptions=False)
 
     def get_name(self, sha1, pattern=None):
         """