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)

0 files changed, 0 insertions, 0 deletions