CI: compare code coverage changes

Also try to enable fast failures experimentally. The number of jobs is
growing, so let's try to keep latencies manageable.

Change-Id: I1c3b54e29e1f2c1c1eed34de18b2343f17f05e64
diff --git a/.zuul.yaml b/.zuul.yaml
index 6f09b90..07833bf 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -1,10 +1,19 @@
 - project:
     check:
+      fail-fast: true
       jobs:
         - clang-format:
             voting: false
-        - f29-gcc:
+        - f29-gcc-cover:
             requires: CzechLight-deps-f29-gcc
+            provides: coverage-f29-gcc
+        - f29-gcc-cover-diff:
+            voting: false
+            requires:
+              - CzechLight-deps-f29-gcc
+              - coverage-f29-gcc
+            dependencies:
+              - f29-gcc-cover
         - f29-gcc-asan-ubsan:
             requires: CzechLight-deps-f29-gcc-asan-ubsan
         - f29-clang-asan: