ci: run style unit tests only if we target develop (#27472)

Some tests assume the base branch is develop, but this branch may not
have been checked out.

lib/spack/spack/test/cmd/style.py

@@ -24,8 +24,19 @@
 
 style = spack.main.SpackCommand("style")
 
+
+def has_develop_branch():
+    git = which('git')
+    if not git:
+        return False
+    git("show-ref", "--verify", "--quiet",
+        "refs/heads/develop", fail_on_error=False)
+    return git.returncode == 0
+
+
 # spack style requires git to run -- skip the tests if it's not there
-pytestmark = pytest.mark.skipif(not which('git'), reason='requires git')
+pytestmark = pytest.mark.skipif(not has_develop_branch(),
+                                reason='requires git with develop branch')
 
 # The style tools have requirements to use newer Python versions.  We simplify by
 # requiring Python 3.6 or higher to run spack style.

lib/spack/spack/test/package_sanity.py

@@ -206,8 +206,12 @@ def test_prs_update_old_api():
     """Ensures that every package modified in a PR doesn't contain
     deprecated calls to any method.
     """
+    ref = os.getenv("GITHUB_BASE_REF")
+    if not ref:
+        pytest.skip("No base ref found")
+
     changed_package_files = [
-        x for x in style.changed_files() if style.is_package(x)
+        x for x in style.changed_files(base=ref) if style.is_package(x)
     ]
     failing = []
     for file in changed_package_files: