gen_pipeline should know of 6X_STABLE branch

It can handle defaults more correctly.
Co-authored-by: NOliver Albertini <oalbertini@pivotal.io>
Co-authored-by: NKris Macoskey <kmacoskey@pivotal.io>
Co-authored-by: NAmil Khanzada <akhanzada@pivotal.io>

concourse/pipelines/gen_pipeline.py

@@ -99,7 +99,7 @@ def suggested_git_branch():
 
     branch = subprocess.check_output("git rev-parse --abbrev-ref HEAD", shell=True).rstrip()
 
-    if branch == "master" or branch == "5X_STABLE":
+    if branch == "master" or branch == "5X_STABLE" or branch == "6X_STABLE":
         return default_branch
     else:
         return branch