ArchiveCommand: Create prefix entry with commit time

ArchiveCommand: Create prefix entry with commit time

The cgit archive command creates a prefix (root) directory entry
in the archive file. That entry's time is set to the commit time.

This patch makes jgit's behavior consistent with with cgit:

prefix: hoge/     -> creates prefix directory "hoge/" entry.
prefix: hoge////  -> creates prefix directory "hoge/" entry.
prefix: hoge/foo  -> does not create prefix directory entry, but for
                     each file/directory entry, prefix is added.

Change-Id: I2610e40ce37972c5f7456fdca6337e7fb07176e5
Signed-off-by: Yasuhiro Takagi <ytakagi@bea.hi-ho.ne.jp>