use xcalloc() to allocate zero-initialized memory

Use xcalloc() instead of xmalloc() followed by memset() to allocate
and zero out memory because it's shorter and avoids duplicating the
function parameters.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
use xmemdupz() to allocate copies of strings given by start and length

Use xmemdupz() to allocate the memory, copy the data and make sure to
NUL-terminate the result, all in one step.  The resulting code is
shorter, doesn't contain the constants 1 and '\0', and avoids
duplicating function parameters.

For blame, the last copied byte (o->file.ptr[o->file.size]) is always
set to NUL by fake_working_tree_commit() or read_sha1_file(), so no
information is lost by the conversion to using xmemdupz().

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>