Set commit time to ZipArchiveEntry

Set commit time to ZipArchiveEntry

Archived zip files for a same commit have different MD5 hash because
mdate and mdate in the header of zip entries are not specified. In
this case, Commons Compress sets an archived time.

In the original git implementation, it's set a commit time:
https://github.com/git/git/blob/e2b2d6a172b76d44cb7b1ddb12ea5bfac9613a44/archive.c#L378

By this fix, archive command sets the commit time to ZipArchiveEntry
when RevCommit is given as an archiving target.

Change-Id: I30dd8710e910cdf42d57742f8709e9803930a123
Signed-off-by: Naoki Takezoe <takezoe@gmail.com>
Signed-off-by: David Pursehouse <david.pursehouse@gmail.com>
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>