ci: centralize matrix vars in init job

Define runners and git-versions once in an init job, then reference
via fromJSON() in both build-git and test matrices. Avoids duplication
and keeps version justification comments in a single place.

Author: clemlesne

.github/workflows/ci.yml

@@ -6,24 +6,30 @@ on:
   workflow_dispatch:
 name: CI
 jobs:
+  init:
+    runs-on: ubuntu-latest
+    outputs:
+      runners: ${{ steps.matrix.outputs.runners }}
+      git-versions: ${{ steps.matrix.outputs.git-versions }}
+    steps:
+      - name: Define matrix
+        id: matrix
+        run: |
+          echo 'runners=["ubuntu-24.04","ubuntu-22.04","ubuntu-24.04-arm","ubuntu-22.04-arm","macos-15","macos-26","macos-15-intel","macos-26-intel"]' >> "$GITHUB_OUTPUT"
+          # Git versions to test against:
+          #   2.34.1 — Ubuntu 22.04 LTS (Jammy), oldest supported LTS, ships 1:2.34.1-1ubuntu1
+          #   2.39.5 — Debian 12 (Bookworm), ships 1:2.39.5-0+deb12u3, reported failing in #1029
+          #   2.47.3 — Debian 13 (Trixie) + Alpine 3.21, ships 1:2.47.3-0+deb13u1, also failing in #1029
+          #   2.53.0 — Latest stable (2026-02-02), first version with cat-file --batch --filter= (added in 2.50.0)
+          echo 'git-versions=["2.34.1","2.39.5","2.47.3","2.53.0"]' >> "$GITHUB_OUTPUT"
+
   build-git:
+    needs: init
     strategy:
       fail-fast: false
       matrix:
-        runner:
-          - ubuntu-24.04
-          - ubuntu-22.04
-          - ubuntu-24.04-arm
-          - ubuntu-22.04-arm
-          - macos-15
-          - macos-26
-          - macos-15-intel
-          - macos-26-intel
-        git-version:
-          - "2.34.1" # Ubuntu 22.04 LTS (Jammy) — oldest supported LTS, ships 1:2.34.1-1ubuntu1
-          - "2.39.5" # Debian 12 (Bookworm) — ships 1:2.39.5-0+deb12u3, reported failing in #1029
-          - "2.47.3" # Debian 13 (Trixie) + Alpine 3.21 — ships 1:2.47.3-0+deb13u1, also failing in #1029
-          - "2.53.0" # Latest stable (2026-02-02) — first version in matrix with cat-file --batch --filter= (added in 2.50.0)
+        runner: ${{ fromJSON(needs.init.outputs.runners) }}
+        git-version: ${{ fromJSON(needs.init.outputs.git-versions) }}
     runs-on: ${{ matrix.runner }}
     name: build-git (${{ matrix.runner }}, git ${{ matrix.git-version }})
     steps:
@@ -74,24 +80,12 @@ jobs:
           retention-days: 1
 
   test:
-    needs: build-git
+    needs: [init, build-git]
     strategy:
       fail-fast: false
       matrix:
-        runner:
-          - ubuntu-24.04
-          - ubuntu-22.04
-          - ubuntu-24.04-arm
-          - ubuntu-22.04-arm
-          - macos-15
-          - macos-26
-          - macos-15-intel
-          - macos-26-intel
-        git-version:
-          - "2.34.1" # Ubuntu 22.04 LTS (Jammy) — oldest supported LTS, ships 1:2.34.1-1ubuntu1
-          - "2.39.5" # Debian 12 (Bookworm) — ships 1:2.39.5-0+deb12u3, reported failing in #1029
-          - "2.47.3" # Debian 13 (Trixie) + Alpine 3.21 — ships 1:2.47.3-0+deb13u1, also failing in #1029
-          - "2.53.0" # Latest stable (2026-02-02) — first version in matrix with cat-file --batch --filter= (added in 2.50.0)
+        runner: ${{ fromJSON(needs.init.outputs.runners) }}
+        git-version: ${{ fromJSON(needs.init.outputs.git-versions) }}
     runs-on: ${{ matrix.runner }}
     name: test (${{ matrix.runner }}, git ${{ matrix.git-version }})
     steps: