Git.pm: use "rev-parse --absolute-git-dir" rather than perl code

When we open a repository with the "Directory" option, we use "rev-parse
--git-dir" to get the path relative to that directory, and then use
Cwd::abs_path() to make it absolute (since our process working directory
may not be the same).

These days we can just ask for "--absolute-git-dir" instead, which saves
us a little code. That option was added in Git v2.13.0 via a2f5a87
(rev-parse: add '--absolute-git-dir' option, 2017-02-03). I don't think
we make any promises about running mismatched versions of git and
Git.pm, but even if somebody tries it, that's sufficiently old that it
should be OK.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: peff

perl/Git.pm

@@ -187,7 +187,7 @@ sub repository {
 		try {
 		  # Note that "--is-bare-repository" must come first, as
 		  # --git-dir output could contain newlines.
-		  $out = $search->command([qw(rev-parse --is-bare-repository --git-dir)],
+		  $out = $search->command([qw(rev-parse --is-bare-repository --absolute-git-dir)],
 			                  STDERR => 0);
 		} catch Git::Error::Command with {
 			throw Error::Simple("fatal: not a git repository: $opts{Directory}");
@@ -196,12 +196,12 @@ sub repository {
 		chomp $out;
 		my ($bare, $dir) = split /\n/, $out, 2;
 
-		require Cwd;
-		require File::Spec;
-		File::Spec->file_name_is_absolute($dir) or $dir = $opts{Directory} . '/' . $dir;
-		$opts{Repository} = Cwd::abs_path($dir);
+		# We know this is an absolute path, because we used
+		# --absolute-git-dir above.
+		$opts{Repository} = $dir;
 
 		if ($bare ne 'true') {
+			require Cwd;
 			# If --git-dir went ok, this shouldn't die either.
 			my $prefix = $search->command_oneline('rev-parse', '--show-prefix');
 			$dir = Cwd::abs_path($opts{Directory}) . '/';