[PATCH] perl/Git.pm: add parse_rev method

@@ -716,6 +716,44 @@ sub ident_person {
 	return "$ident[0] <$ident[1]>";
 }
 
+=item parse_rev ( REVISION_NAME )
+
+Look up the specified revision name and return the SHA1 hash, or
+return undef if the lookup failed.  See git rev-parse --help, section
+"Specifying Revisions".
+
+=cut
+
+sub parse_rev {
+    # We could allow for a list of revisions here.
+    my ($self, $rev_name) = @_;
+
+    my $hash;
+    try {
+        # The --quiet --verify options cause git-rev-parse to fail
+        # with exit status 1 (instead of 128) if the given revision
+        # name is not found, which enables us to distinguish not-found
+        # from serious errors.  The --default option works around
+        # git-rev-parse's lack of support for getopt style "--"
+        # separators (it would fail for tags named "--foo" without
+        # it).
+        $hash = $self->command_oneline("rev-parse", "--verify", "--quiet",
+                                       "--default", $rev_name);
+    } catch Git::Error::Command with {
+        my $E = shift;
+        if ($E->value() == 1) {
+            # Revision name not found.
+            $hash = undef;
+        } else {
+            throw $E;
+        }
+    };
+    # Guard against unexpected output.
+    throw Error::Simple(
+        "parse_rev: unexpected output for \"$rev_name\": $hash")
+        if defined $hash and $hash !~ /^([0-9a-fA-F]{40})$/;
+    return $hash;
+}
 
 =item hash_object ( TYPE, FILENAME )