Introduce basic incremental mutation testing

We introduce a CI job for mutation testing of PR diffs using cargo-mutants.
Missed cases do not trigger a fail of this job yet as we just introduce it
now for visibility. We may start enforcing stricter rules at a later stage.

Author: dunxen

.github/workflows/build.yml

@@ -247,3 +247,19 @@ jobs:
           rustup component add rustfmt
       - name: Run rustfmt checks
         run: ci/rustfmt.sh
+
+  incremental-mutants:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+        with:
+          fetch-depth: 0
+      - name: Relative diff
+        run: |
+          git branch -av
+          git diff origin/main.. | tee git.diff
+      - uses: Swatinem/rust-cache@v2
+      - name: Mutants
+        run: |
+          cargo install cargo-mutants
+          cargo mutants --no-shuffle -j 2 -vV --in-diff git.diff

lightning/src/ln/channel.rs

@@ -7522,6 +7522,8 @@ impl<SP: Deref> OutboundV1Channel<SP> where SP::Target: SignerProvider {
 		if !self.context.is_outbound() {
 			panic!("Tried to create outbound funding_created message on an inbound channel!");
 		}
+		// REMOVE ME - TRIGGER MUTANTS
+		if is_batch_funding {};
 		if !matches!(
 			self.context.channel_state, ChannelState::NegotiatingFunding(flags)
 			if flags == (NegotiatingFundingFlags::OUR_INIT_SENT | NegotiatingFundingFlags::THEIR_INIT_SENT)