buildman: Don't prune output space for 'current source' build

This is not needed since we always do a full (non-incremental) build. Also
it might be dangerous since it will try to delete everything below the
base directory.

Fix this potentially nasty bug.

Signed-off-by: Simon Glass <sjg@chromium.org>

tools/buildman/builder.py

@@ -1115,6 +1115,8 @@ class Builder:
         create. Having left over directories is confusing when the user wants
         to check the output manually.
         """
+        if not self.commits:
+            return
         dir_list = []
         for commit_upto in range(self.commit_count):
             dir_list.append(self._GetOutputDir(commit_upto))