bots: Add make-checkout option to checkout a different cockpit bots/ ref

For testing external projects for changes to cockpit's bots/ directory,
we need to be able to check out the project plus Cockpit's bots/ from a
ref other than master (i. e. usually a pull request, like
pull/1234/head). Add a new `--bots-ref` option to it which does that.
When not given, bots/ will continue to be checked out from
`origin/master`.

Closes cockpit-project#9605

bots/make-checkout

@@ -36,6 +36,7 @@ def main():
     parser.add_argument("-v", "--verbose", action="store_true", help="Verbose output")
     parser.add_argument("--base", nargs='?', help="Base branch that revision is for")
     parser.add_argument("--repo", nargs='?', help="Repository to check out")
+    parser.add_argument("--bots-ref", help="bots/ ref to check out from cockpit, for non-default --repo")
     parser.add_argument("ref", help="The git ref to fetch")
     parser.add_argument("revision", nargs='?', help="Actual commit to check out, defaults to ref")
 
@@ -54,6 +55,13 @@ def main():
             sys.stderr.write("> " + output + "\n")
         return output
 
+    # do this before fetching the actual test repo, so that FETCH_HEAD always points to that at the end
+    if opts.bots_ref:
+        execute("git", "fetch", "origin", opts.bots_ref)
+        bots_ref = execute("git", "rev-parse", "FETCH_HEAD").strip()
+    else:
+        bots_ref = "origin/master"
+
     if opts.repo:
         if "test\n" in subprocess.check_output([ "git", "remote" ], universal_newlines=True):
             execute("git", "remote", "remove", "test")
@@ -64,8 +72,8 @@ def main():
 
     # If the bots directory doesn't exist in this branch or repo, check it out from master
     if opts.repo or (opts.base and opts.base != "master"):
-        sys.stderr.write("Checking out bots directory from Cockpit master ...\n")
-        execute("git", "checkout", "--force", "origin/master", "--", "bots/")
+        sys.stderr.write("Checking out bots directory from Cockpit %s ...\n" % (opts.bots_ref or bots_ref))
+        execute("git", "checkout", "--force", bots_ref, "--", "bots/")
 
 
 if __name__ == '__main__':