buildslave/git: Don't fail the whole build if git branch -M can't ren…

…ame master.

Previous versions of Git allowed the current branch to be renamed. For
some reason, Git 1.7.7 does not allow this. All attempts to rename the
current branch fail with "Cannot force update the current branch." So,
if the rename fails, don't abort.

As part of this, add support for commands that can fail.

slave/buildslave/commands/git.py

@@ -63,7 +63,7 @@ def _fullSrcdir(self):
     def sourcedirIsUpdateable(self):
         return os.path.isdir(os.path.join(self._fullSrcdir(), ".git"))
 
-    def _dovccmd(self, command, cb=None, **kwargs):
+    def _dovccmd(self, command, cb=None, stopOnFail=True, **kwargs):
         git = self.getCommand("git")
         c = runprocess.RunProcess(self.builder, [git] + command, self._fullSrcdir(),
                          sendRC=False, timeout=self.timeout,
@@ -72,7 +72,8 @@ def _dovccmd(self, command, cb=None, **kwargs):
         self.command = c
         d = c.start()
         if cb:
-            d.addCallback(self._abandonOnFailure)
+            if stopOnFail:
+                d.addCallback(self._abandonOnFailure)
             d.addCallback(cb)
         return d
 
@@ -107,7 +108,7 @@ def _didHeadCheckout(self, res):
         # Rename branch, so that the repo will have the expected branch name
         # For further information about this, see the commit message
         command = ['branch', '-M', self.branch]
-        return self._dovccmd(command, self._initSubmodules)
+        return self._dovccmd(command, self._initSubmodules, False)
 
     def _didFetch(self, res):
         if self.revision: