write_entry(): separate two helper functions out

write_entry(): separate two helper functions out

In the write-out codepath, a block of code determines what file in the
working tree to write to, and opens an output file descriptor to it.

After writing the contents out to the file, another block of code runs
fstat() on the file descriptor when appropriate.

Separate these blocks out to open_output_fd() and fstat_output()
helper functions.

Signed-off-by: Junio C Hamano <gitster@pobox.com>