msvc: Fix an "conversion, loss of data" compiler warning

In particular, the compiler issues the following warning:

    src/revwalk.c(61) : warning C4244: '=' : conversion from \
        'unsigned int' to 'unsigned char', possible loss of data

In order to suppress the warning, we change the type of the
sorting "enum" field of the git_revpool structure to be consistent
with the sort_mode parameter of the gitrp_sorting() function.

Note that if the size of the git_revpool structure is an issue,
then we could change the type of the sort_mode parameter instead.

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Andreas Ericsson <ae@op5.se>

src/revwalk.h

@@ -14,7 +14,7 @@ struct git_revpool {
 	git_revpool_table *commits;
 
 	unsigned walking:1;
-	unsigned char sorting;
+	unsigned int sorting;
 };
 
 void gitrp__prepare_walk(git_revpool *pool);