fast-import.c: ignorecase iff explicitly told to

fast-import *should* import exactly what it's told to import down
to the exact case of the filenames.  If case folding is enabled,
the import will not produce the same commit ids, so we don't want
git to fold case by default.

This change adds the --ignore-case option to git-fast-import.  The
default is to not ignore the case, IOW, to honor the case.