write_entry(): cleanup of some duplicated code

The switch-cases for S_IFREG and S_IFLNK was so similar that it will
be better to do some cleanup and use the common parts of it.

And the entry.c file should now be clean for 'gcc -Wextra' warnings.

Signed-off-by: Kjetil Barvik <barvik@broadpark.no>
Signed-off-by: Junio C Hamano <gitster@pobox.com>