In reference_read we stat a file and then call futils which stats it
again. Use git_futils_readbuffer_updated to avoid the extra stat
call. This introduces another parameter which is used to tell the
caller whether the file was read or not.

Modify the callers to take advantage of this new feature. This change
removes ~140 stat calls from the test suite.

Signed-off-by: Carlos Martín Nieto <cmn@elego.de>