Use os.path.join for git driver getGitUrl

The current code uses a format string with an unconditional injection
of a /. This can result in a double-slash which can then cause git to
be unable to clone things. That in turn can lead to an indefinite hang
of a job trying to use the repo from the git driver.

Replace the format string with os.path.join which will correctly join
the project to the baseurl whether it's a url or a bare path, and
whether or not it has a trailing slash.

Change-Id: Id0e529b916ff75d6b6df98d978d2349f6037499c

releasenotes/notes/git-baseurl-hang-fe1ffbf696940a2b.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    Fixed an issue where a trailing slash on the baseurl of a git driver
+    connection config could cause an indefinite job hang.

zuul/driver/git/gitconnection.py

@@ -231,8 +231,7 @@ class GitConnection(BaseConnection):
         return branches
 
     def getGitUrl(self, project):
-        url = '%s/%s' % (self.baseurl, project.name)
-        return url
+        return os.path.join(self.baseurl, project.name)
 
     def onLoad(self):
         self.log.debug("Starting Git Watcher")