Change tree pruning algorithm

The first merge we see in Factory determines if we keep the devel
commits in the factory chain or cut that branch.

tests/fixtures/clapper-expected-tree.yaml

@@ -4,4 +4,9 @@
 - commit: openSUSE:Factory/clapper/2.0
   merged:
   - multimedia:apps/clapper/7.0
+  - multimedia:apps/clapper/6.0
+  - multimedia:apps/clapper/5.0
 - commit: openSUSE:Factory/clapper/1.0
+- commit: multimedia:apps/clapper/3.0
+- commit: multimedia:apps/clapper/2.0
+- commit: multimedia:apps/clapper/1.0

tests/fixtures/llvm13-expected-tree.yaml

@@ -44,4 +44,6 @@
 - commit: openSUSE:Factory/llvm13/2.0
   merged:
   - devel:tools:compiler/llvm13/4.0
+  - devel:tools:compiler/llvm13/3.0
 - commit: openSUSE:Factory/llvm13/1.0
+- commit: devel:tools:compiler/llvm13/1.0

tests/fixtures/zsh-expected-tree.yaml

@@ -286,6 +286,8 @@
 - commit: openSUSE:Factory/zsh/13.0
   merged:
   - shells/zsh/34.0
+  - shells/zsh/33.0
+  - shells/zsh/32.0
 - commit: openSUSE:Factory/zsh/12.0
 - commit: openSUSE:Factory/zsh/11.0
 - commit: openSUSE:Factory/zsh/10.0