avoid creating empty files when one side of a diff is empty; ok tracey

create temporary workaround to opening /dev/null during diffing. this will need to be reworked later. ok stsp

Add missing includes for time() and ctime() These headers were implicitly used on OpenBSD, but building on Linux revealed that <time.h> wasn't included directly. diffreg.c uses ctime(), which additionally needs <sys/types.h>.

Remove return value from diffreg's fetch() The last use of this was removed in 0165cdf03ffac4c24334979832ad27554913d54c over a year ago.

Remove unused variable in diffreg's fetch()

remove unused 'oldfile' parameter from diffreg.c's fetch()

initial 'got stage -p' implementation

remove accidental assignment in diffreg.c

plug /dev/null file descriptor leaks in diffreg.c

rename got_error_prefix_errno() to got_error_from_errno()

got_error_from_errno -> got_error_prefix_errno also add got_error_prefix_errno2 and got_error_prefix_errno3 which should hopefully all be merged into a single function with variadic args (but can't alloc mem)

don't close input files in got_diffreg(); callers will do that

check for errors from fclose()

restore support for D_NORMAL diffs of regular files

use a constant for max diff context size