Preallocate memory earlier in fast-import

Preallocate memory earlier in fast-import

I'm about to teach fast-import how to reload the marks file created
by a prior session.  The general approach that I want to use is to
immediately parse the marks file when the specific argument is found
in argv, thereby allowing the caller to supply multiple marks files,
as the mark space can be sparsely populated.

To make that work out we need to allocate our object tables before
we parse the command line options.  Since none of these tables
depend on the command line options, we can easily relocate them.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>