compat: helper for detecting unsigned overflow

compat: helper for detecting unsigned overflow

The idiom (a + b < a) works fine for detecting that an unsigned
integer has overflowed, but a more explicit

	unsigned_add_overflows(a, b)

might be easier to read.

Define such a macro, expanding roughly to ((a) < UINT_MAX - (b)).
Because the expansion uses each argument only once outside of sizeof()
expressions, it is safe to use with arguments that have side effects.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>