unary minus operator applied to unsigned type, result still unsigned?

I'm working on building Git on Visual Studio as part of the Git for Windows
capability.

The MSVC compiler is reporting:

1>..\sha1-lookup.c(100) : warning C4146: unary minus operator applied to
unsigned type, result still unsigned

1>..\sha1-lookup.c(316) : warning C4146: unary minus operator applied to
unsigned type, result still unsigned

the two lines of code are the same, and the message suggests a bad return 
value:

#100: return -lo-1;

#316: return -lo-1;


Should these be protected by an appropriate brackets/calculation (-1-lo) ? 
Or does
gcc use an alternate assumption about unary minus conversion for functions
returning int?

--

Philip