Make tests work with detached git dir

In my jenkins replacement, I run all tests with .git outside the work tree,
pointed to by $GIT_DIR. This is fairly common git practice, so let's make the
testsuite support this and run the relevant porting tests that are skipped
without this patch.

Porting/cmpVERSION.pl

@@ -32,7 +32,7 @@
 die "$0: This does not look like a Perl directory\n"
     unless -f "perl.h" && -d "Porting";
 die "$0: 'This is a Perl directory but does not look like Git working directory\n"
-    unless -d ".git";
+    unless (-d ".git" || (exists $ENV{GIT_DIR} && -d $ENV{GIT_DIR}));
 
 my $null = devnull();
 

t/test.pl

@@ -176,6 +176,13 @@ sub find_git_or_skip {
 	    }
 	    $source_dir = $where;
 	}
+    } elsif (exists $ENV{GIT_DIR}) {
+	my $commit = '8d063cd8450e59ea1c611a2f4f5a21059a2804f1';
+	my $out = `git rev-parse --verify --quiet '$commit^{commit}'`;
+	chomp $out;
+	if($out eq $commit) {
+	    $source_dir = '.'
+	}
     }
     if ($source_dir) {
 	my $version_string = `git --version`;