Attempts to write entries with too-long filenames currently error out:

  $ jgit.pgm/target/jgit archive HEAD >test.tar
  java.lang.RuntimeException: file name 'org.eclipse.jgit.http.server/src/org/eclipse/jgit/http/server/resolver/DefaultReceivePackFactory.java' is too long ( > 100 bytes)
          at org.apache.commons.compress.archivers.tar.TarArchiveOutputStream.putArchiveEntry(TarArchiveOutputStream.java:288)
          at org.eclipse.jgit.archive.TarFormat.putEntry(TarFormat.java:92)
          at org.eclipse.jgit.archive.TarFormat.putEntry(TarFormat.java:62)
          at org.eclipse.jgit.api.ArchiveCommand.writeArchive(ArchiveCommand.java:293)
          at org.eclipse.jgit.api.ArchiveCommand.call(ArchiveCommand.java:322)
          at org.eclipse.jgit.pgm.Archive.run(Archive.java:97)
          at org.eclipse.jgit.pgm.TextBuiltin.execute(TextBuiltin.java:174)
          at org.eclipse.jgit.pgm.Main.execute(Main.java:213)
          at org.eclipse.jgit.pgm.Main.run(Main.java:121)
          at org.eclipse.jgit.pgm.Main.main(Main.java:95)

That's because the default longFileMode is LONGFILE_ERROR, which
throws an exception for filenames longer than 100 characters.  Switch
to LONGFILE_POSIX.  While at it, handle large files and filenames with
strange encodings, too.

This requires commons compress 1.4, which introduced support for large
files and POSIX long filenames.

Change-Id: I04d5427eec0968b129f55d7a4c6021039a494828

First step toward making ArchiveCommand itself format-agnostic.

Change-Id: I3cff5fce28fa7a19e34f8291cfb5b62f16429713

Unlike ZIP files, tar files do not treat symlinks as ordinary files
with a different mode, so tar support involves a little more code than
would be ideal.

Change-Id: Ica2568f4a0e443bf4b955ef0c029bc8eec62d369

Setting the mode for a zip entry is now as simple as
"entry.setUnixMode(mode)", so do that.

The test checks using the system's "zipinfo" command (from InfoZIP)
that the mode has been recorded correctly on systems that happen to
have a "zipinfo" command, using org.junit.Assume to distinguish them.

Change-Id: I4236c102fd76f18d01b2dc926eeb9b9fa11a61b7

C Git's "git archive" command represents a tree object using a
standard archival format like tar, zip, or tgz, ready for consumption
by other, git-unaware users or tools.

Add a bare-bones analagous "jgit archive" command to show what is
possible, supporting only ZIP format for now.  It uses java.util.zip
which is not aware of the InfoZIP extensions for representing symlinks
and file permissions, so symlinks, executable files, and submodule
entries are represented as plain text files.

Making this functionality available from the library, improving
handling of special entries, and support for other output formats are
left for later patches.  Ultimately the intent is to offer a
TreeArchiveStream class for use by web frontends like Gitiles to offer
"download as zip/tgz/txz" links and use by, for example, code search
tools to get easy access to the content of git tree objects.

Test with "jgit archive my-favorite-tree >out.zip".

Change-Id: Ib590f173ceff3df4b58493cecccd6b9a1b355e3d