Commit sha is need for diff comparisons

* master is fine for locally checked-out branch

* for PRs there's no master label poiting to a specific base branch sha,
  but there's a sha handle coming from github event

Renaming to make more sense describing the actual value.

Author: mostlyobvious

.github/workflows/aggregate_root_mutate.yml

@@ -39,7 +39,7 @@ jobs:
       RUBY_VERSION: "${{ matrix.ruby_version }}"
       BUNDLE_GEMFILE: "${{ matrix.bundle_gemfile }}"
       BUNDLE_WITHOUT: database
-      BASE_REF: "${{ github.base_ref || 'HEAD~1' }}"
+      SINCE_SHA: "${{ github.event.pull_request.base.sha || 'HEAD~1' }}"
     strategy:
       fail-fast: false
       matrix:

.github/workflows/minitest-ruby_event_store_mutate.yml

@@ -29,7 +29,7 @@ jobs:
       RUBY_VERSION: "${{ matrix.ruby_version }}"
       BUNDLE_GEMFILE: "${{ matrix.bundle_gemfile }}"
       BUNDLE_WITHOUT: database
-      BASE_REF: "${{ github.base_ref || 'HEAD~1' }}"
+      SINCE_SHA: "${{ github.event.pull_request.base.sha || 'HEAD~1' }}"
     strategy:
       fail-fast: false
       matrix:

.github/workflows/rails_event_store_mutate.yml

@@ -39,7 +39,7 @@ jobs:
       RUBY_VERSION: "${{ matrix.ruby_version }}"
       BUNDLE_GEMFILE: "${{ matrix.bundle_gemfile }}"
       BUNDLE_WITHOUT: database
-      BASE_REF: "${{ github.base_ref || 'HEAD~1' }}"
+      SINCE_SHA: "${{ github.event.pull_request.base.sha || 'HEAD~1' }}"
     strategy:
       fail-fast: false
       matrix:

.github/workflows/ruby_event_store-active_record_mutate.yml

@@ -39,7 +39,7 @@ jobs:
       RUBY_VERSION: "${{ matrix.ruby_version }}"
       BUNDLE_GEMFILE: "${{ matrix.bundle_gemfile }}"
       BUNDLE_WITHOUT: database
-      BASE_REF: "${{ github.base_ref || 'HEAD~1' }}"
+      SINCE_SHA: "${{ github.event.pull_request.base.sha || 'HEAD~1' }}"
     strategy:
       fail-fast: false
       matrix:

.github/workflows/ruby_event_store-browser_mutate.yml

@@ -39,7 +39,7 @@ jobs:
       RUBY_VERSION: "${{ matrix.ruby_version }}"
       BUNDLE_GEMFILE: "${{ matrix.bundle_gemfile }}"
       BUNDLE_WITHOUT: database
-      BASE_REF: "${{ github.base_ref || 'HEAD~1' }}"
+      SINCE_SHA: "${{ github.event.pull_request.base.sha || 'HEAD~1' }}"
     strategy:
       fail-fast: false
       matrix:

.github/workflows/ruby_event_store-flipper_mutate.yml

@@ -29,7 +29,7 @@ jobs:
       RUBY_VERSION: "${{ matrix.ruby_version }}"
       BUNDLE_GEMFILE: "${{ matrix.bundle_gemfile }}"
       BUNDLE_WITHOUT: database
-      BASE_REF: "${{ github.base_ref || 'HEAD~1' }}"
+      SINCE_SHA: "${{ github.event.pull_request.base.sha || 'HEAD~1' }}"
     strategy:
       fail-fast: false
       matrix:

.github/workflows/ruby_event_store-newrelic_mutate.yml

@@ -29,7 +29,7 @@ jobs:
       RUBY_VERSION: "${{ matrix.ruby_version }}"
       BUNDLE_GEMFILE: "${{ matrix.bundle_gemfile }}"
       BUNDLE_WITHOUT: database
-      BASE_REF: "${{ github.base_ref || 'HEAD~1' }}"
+      SINCE_SHA: "${{ github.event.pull_request.base.sha || 'HEAD~1' }}"
     strategy:
       fail-fast: false
       matrix:

.github/workflows/ruby_event_store-outbox_mutate.yml

@@ -29,7 +29,7 @@ jobs:
       RUBY_VERSION: "${{ matrix.ruby_version }}"
       BUNDLE_GEMFILE: "${{ matrix.bundle_gemfile }}"
       BUNDLE_WITHOUT: database
-      BASE_REF: "${{ github.base_ref || 'HEAD~1' }}"
+      SINCE_SHA: "${{ github.event.pull_request.base.sha || 'HEAD~1' }}"
     strategy:
       fail-fast: false
       matrix:

.github/workflows/ruby_event_store-profiler_mutate.yml

@@ -29,7 +29,7 @@ jobs:
       RUBY_VERSION: "${{ matrix.ruby_version }}"
       BUNDLE_GEMFILE: "${{ matrix.bundle_gemfile }}"
       BUNDLE_WITHOUT: database
-      BASE_REF: "${{ github.base_ref || 'HEAD~1' }}"
+      SINCE_SHA: "${{ github.event.pull_request.base.sha || 'HEAD~1' }}"
     strategy:
       fail-fast: false
       matrix:

.github/workflows/ruby_event_store-protobuf_mutate.yml

@@ -29,7 +29,7 @@ jobs:
       RUBY_VERSION: "${{ matrix.ruby_version }}"
       BUNDLE_GEMFILE: "${{ matrix.bundle_gemfile }}"
       BUNDLE_WITHOUT: database
-      BASE_REF: "${{ github.base_ref || 'HEAD~1' }}"
+      SINCE_SHA: "${{ github.event.pull_request.base.sha || 'HEAD~1' }}"
     strategy:
       fail-fast: false
       matrix: