Fix compilation warnings in MSVC

This allows to successfully build libgit2 with waf on Windows.

src/config_file.c

@@ -638,7 +638,7 @@ static int parse_section_header_ext(const char *line, const char *base_name, cha
 			break;
 		}
 
-		subsection[pos++] = c;
+		subsection[pos++] = (char) c;
 	} while ((c = line[rpos++]) != ']');
 
 	subsection[pos] = '\0';
@@ -719,7 +719,7 @@ static int parse_section_header(diskfile_backend *cfg, char **section_out)
 			goto error;
 		}
 
-		name[name_length++] = tolower(c);
+		name[name_length++] = (char) tolower(c);
 
 	} while ((c = line[pos++]) != ']');
 

src/util.c

@@ -109,7 +109,7 @@ void git__strntolower(char *str, int len)
 	int i;
 
 	for (i = 0; i < len; ++i) {
-		str[i] = tolower(str[i]);
+		str[i] = (char) tolower(str[i]);
 	}
 }