ci: collect test coverage and deploy a html report through gitlab pages

Use lcov for both Fedora and MSYS2 to create coverage reports and add a second
ci stage which merges the coverage and creates a html report using genhtml.

In the final stage, which is only run on master, the result is published on
gitlab pages.

https://bugzilla.gnome.org/show_bug.cgi?id=795636

.gitlab-ci/fixup-cov-paths.py

@@ -0,0 +1,29 @@
+import sys
+import os
+import io
+
+
+def main(argv):
+    # Fix paths in lcov files generated on a Windows host so they match our
+    # current source layout.
+    paths = argv[1:]
+
+    for path in paths:
+        print("cov-fixup:", path)
+        text = io.open(path, "r", encoding="utf-8").read()
+        text = text.replace("\\\\", "/")
+        glib_dir = "/glib/"
+        end = text.index(glib_dir)
+        start = text[:end].rindex(":") + 1
+        old_root = text[start:end]
+        assert os.path.basename(os.getcwd()) == "glib"
+        new_root = os.path.dirname(os.getcwd())
+        if old_root != new_root:
+            print("replacing %r with %r" % (old_root, new_root))
+        text = text.replace(old_root, new_root)
+        with io.open(path, "w", encoding="utf-8") as h:
+            h.write(text)
+
+
+if __name__ == "__main__":
+    sys.exit(main(sys.argv))