Don't double-include if global-requirements

global-requirements is a fully self-contained file, containing both
test and normal requirements. In the situation where the target repo
has a global-requirements.txt file, it should be treated as the only
file that matters- otherwise the mirror builder will hit double include
issues.

Change-Id: I01fd1e0d7f0923456f35a0f931159850cb52e3e7
diff --git a/jeepyb/cmd/run_mirror.py b/jeepyb/cmd/run_mirror.py
index 5ba2efd..5e54583 100644
--- a/jeepyb/cmd/run_mirror.py
+++ b/jeepyb/cmd/run_mirror.py
@@ -208,13 +208,15 @@
                     out = self.run_command("git reset --hard %s" % branch)
                     out = self.run_command("git clean -x -f -d -q")
                 reqlist = []
-                for requires_file in ("global-requirements.txt",
-                                      "requirements.txt",
-                                      "test-requirements.txt",
-                                      "tools/pip-requires",
-                                      "tools/test-requires"):
-                    if os.path.exists(requires_file):
-                        reqlist.append(requires_file)
+                if os.path.exists('global-requirements.txt'):
+                    reqlist.append('global-requirements.txt')
+                else:
+                    for requires_file in ("requirements.txt",
+                                          "test-requirements.txt",
+                                          "tools/pip-requires",
+                                          "tools/test-requires"):
+                        if os.path.exists(requires_file):
+                            reqlist.append(requires_file)
                 if reqlist:
                     out = self.run_command(venv_format %
                                            (pip_cache_dir, venv))