Initial draft of fast-import documentation.

This is a first pass at the manpage for git-fast-import.

I have tried to cover the input format in extreme detail, creating a
reference which is more detailed than the BNF grammar appearing in
the header of fast-import.c.  I have also covered some details about
gfi's performance and memory utilization, as well as the average
learning curve required to create a gfi frontend application (as it
is far lower than it might appear on first glance).

The documentation still lacks real example input streams, which may
turn out to be difficult to format in asciidoc due to the blank lines
which carry meaning within the format.
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>