largefiles: fix cat for largefiles (issue3352)
This is a fix to largefiles so that 'hg cat' will work correctly when a
largefile is specified.
As per discussion on Issue 3352:
1) The file will be printed regardless if it is binary or large.
2) The file is downloaded if it is not readily available (not found in
the system cache), so that it can be printed. If the download fails,
then we abort.