fix t/porting/authors.t detection of merge commits

git-rev-parse HEAD^2` was used to detect if HEAD was a merge commit.
When HEAD is not a merge commit then HEAD^2 produces an error;
When HEAD is a merge commit then HEAD^2 points to the second parent.

The problem was that `git rev-parse "HEAD^2"` also produced output on STDOUT
which caused `$revision_range` to be set to something bogus.

Example:
    $ git rev-parse HEAD^2 2>/dev/null ; echo $?
    HEAD^2
    128
    $ git log HEAD^1..HEAD^2
    fatal: ambiguous argument 'HEAD^1..HEAD^2': unknown revision or path not in the working tree.
    Use '--' to separate paths from revisions, like this:
    'git <command> [<revision>...] -- [<file>...]'

The fix is to use --verify which doesn't give output on STDOUT for something invalid.
It does print an error on STDERR (for invalid things) which can be silenced using -q
    $ git rev-parse --verify HEAD^2
    fatal: Needed a single revision

    $ git rev-parse -q --verify HEAD^2
    $

For an actual merge commit:
    $ git rev-parse -q --verify e91a97a^2
    bee6601bbf238b5366ce11cd41f6b095bfeebfd8

(i.e. working as expected)

1 files changed, 1 insertions, 3 deletions

diff --git a/t/porting/authors.t b/t/porting/authors.t
index f2de6bdff1..561d7d795a 100644
--- a/t/porting/authors.t
+++ b/t/porting/authors.t
@@ -9,7 +9,6 @@ BEGIN {
 
 use TestInit qw(T);    # T is chdir to the top level
 use strict;
-use File::Spec;
 
 find_git_or_skip('all');
 skip_all(
@@ -30,8 +29,7 @@ elsif( $ENV{GITHUB_ACTIONS} && length $ENV{GITHUB_BASE_REF} ) {
     # https://help.github.com/en/actions/automating-your-workflow-with-github-actions/using-environment-variables
 
     # we should be on a merge commit, but double check
-    my $null = File::Spec->devnull;
-    my $branch_head = `git rev-parse "HEAD^2" 2>$null`;
+    my $branch_head = `git rev-parse -q --verify "HEAD^2"`;
     chomp $branch_head;
 
     # gives the history of the branch being merged, excluding what it is