toolbox/projtool.pl: show objects/info/packs timestamp

toolbox/projtool.pl: show objects/info/packs timestamp

Not all projects will have a bundle.  All non-empty projects should
have at least one pack.  Since the `git update-server-info` command
only runs after new packs have been created and always creates/modifies
the objects/info/packs file, its date reflects the last time any
pack changes have occurred in the project.

When Git "freshens" a pack's date, it does not touch the
objects/info/packs file at all making its timestamp immune to any
"freshening" activity that may occur.

Include this timestamp in the `projtool.pl show` output if the
file exists.  Comparing this date against the `lastgc` timestamp
can quickly show whether or not the project is dirty (or even
"stale").

Since this timestamp is being added to the `projtool show` output,
also take this opportunity to make the bundle timestamp a little
bit more readable (extra spaces) and use that same format for this
new timestamp.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>