Skip t/porting/authors.t on shallow clones

On shallow clones the history is not available.

Before:
    The 'are we on a branch' logic was broken which caused this
    test to assume it was always on a branch which caused it to
    use: `git log HEAD^1..HEAD^2` | perl Porting/checkAUTHORS.pl --tap`.

    That `git log` command returns *no commits* when HEAD is *not* a branch
    and then the test passes.

    In a shallow clone (clone without history) there is only one commit and
    never a merge so this test silently did nothing on a shallow clone.

Intermediate:
    In commit 16dd3f70cc16005d5af7146385733a7c945fb67e the 'are we on a branch'
    logic was fixed and now it does do the right thing.

    But this introduces a new problem: on a shallow clone there is no history
    so there never is a merge commit. But GitHub Actions does *add* a merge
    commit (in some cases) with a different author/committer name/emailaddress
    (one which should not be in AUTHORS).

    Result: the GitHub CI *failed* because they're using shallow clones.

Now:
    When a shallow clone is detected just skip the entire test.
    Before it already did that (by accident) now make this explicit.

Example git log:

    my branch for which a PR was created:
        $ git log --format=format:"%h %aN <%aE> - %s"  --graph  -n25
            * 12a6316 Bram <perl-rt@wizbit.be> - TMP: Add some temporary debugging
            * 8ac37a5 Bram <perl-rt@wizbit.be> - CI: Special case porting test in 'sanity check'
            * 7f36725 Karl Williamson <khw@cpan.org> - makedef: Export certain symbols

    GitHub CI with full git history (i.e. no shallow clone):
        $ git log --format=format:"%h %aN <%aE> - %s"  --graph  -n25
            *   99884eedeb Bram <109858694+bram-perl@users.noreply.github.com> - Merge 12a6316de1a8834f07d76c59480bafc3fdfa0c66 into 7f367253e335e8507638bb2ca1767c0fedbc95d3
            |\
            | * 12a6316de1 Bram <perl-rt@wizbit.be> - TMP: Add some temporary debugging
            | * 8ac37a5a03 Bram <perl-rt@wizbit.be> - CI: Special case porting test in 'sanity check'
            |/
            * 7f367253e3 Karl Williamson <khw@cpan.org> - makedef: Export certain symbols

    GitHub CI without history (i.e. shallow clone):
        $ git log --format=format:"%h %aN <%aE> - %s"  --graph  -n25
            * 99884eede Bram <109858694+bram-perl@users.noreply.github.com> - Merge 12a6316de1a8834f07d76c59480bafc3fdfa0c66 into 7f367253e335e8507638bb2ca1767c0fedbc95d3

    For future reference: the git show info:
        commit 99884eedebb6cccd1596023a94e32eb3aea6f5e8
        Author:     Bram <109858694+bram-perl@users.noreply.github.com>
        Commit:     GitHub <noreply@github.com>

    (I do *not* have that Author email-address configured)

1 files changed, 5 insertions, 1 deletions

diff --git a/t/porting/authors.t b/t/porting/authors.t
index ffe8f636ec..667b38a480 100644
--- a/t/porting/authors.t
+++ b/t/porting/authors.t
@@ -10,11 +10,15 @@ BEGIN {
 use TestInit qw(T);    # T is chdir to the top level
 use strict;
 
-find_git_or_skip('all');
+my $source_dir = find_git_or_skip('all');
 skip_all(
     "This distro may have modified some files in cpan/. Skipping validation.")
   if $ENV{'PERL_BUILD_PACKAGING'};
 
+skip_all(
+    "This is a shallow clone, this test requires history.")
+  if (-e "$source_dir/.git/shallow");
+
 my $revision_range = ''; # could use 'v5.22.0..' as default, no reason to recheck all previous commits...
 if ( $ENV{TRAVIS} && defined $ENV{TRAVIS_COMMIT_RANGE} ) {
 	# travisci is adding a merge commit when smoking a pull request