Teach import-tars about GNU tar's @LongLink extension.

This extension allows GNU tar to process file names in excess of the 100
characters defined by the original tar standard. It does this by faking a
file, named '././@LongLink' containing the true file name, and then adding
the file with a truncated name. The idea is that tar without this
extension will write out a file with the long file name, and write the
contents into a file with truncated name.

Unfortunately, GNU tar does a lousy job at times. When truncating results
in a _directory_ name, it will happily use _that_ as a truncated name for
the file.

An example where this actually happens is gcc-4.1.2, where the full path
of the file WeThrowThisExceptionHelper.java truncates _exactly_ before the
basename. So, we have to support that ad-hoc extension.

This bug was noticed by Chris Riddoch on IRC.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

contrib/fast-import/import-tars.perl

@@ -52,6 +52,25 @@
 			Z8 Z1 Z100 Z6
 			Z2 Z32 Z32 Z8 Z8 Z*', $_;
 		last unless $name;
+		if ($name eq '././@LongLink') {
+			# GNU tar extension
+			if (read(I, $_, 512) != 512) {
+				die ('Short archive');
+			}
+			$name = unpack 'Z257', $_;
+			next unless $name;
+
+			my $dummy;
+			if (read(I, $_, 512) != 512) {
+				die ('Short archive');
+			}
+			($dummy, $mode, $uid, $gid, $size, $mtime,
+			$chksum, $typeflag, $linkname, $magic,
+			$version, $uname, $gname, $devmajor, $devminor,
+			$prefix) = unpack 'Z100 Z8 Z8 Z8 Z12 Z12
+			Z8 Z1 Z100 Z6
+			Z2 Z32 Z32 Z8 Z8 Z*', $_;
+		}
 		next if $name =~ m{/\z};
 		$mode = oct $mode;
 		$size = oct $size;