Fix uninitialized variable in string_expand_length

This led to totally unpredictable results from the function. The style
matches the loop in string_expand.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Jonas Fonseca <fonseca@diku.dk>

tig.c

@@ -193,7 +193,7 @@ string_expand_length(const char *line, int tabsize)
 {
 	size_t size, pos;
 
-	for (pos = 0; line[pos]; pos++) {
+	for (size = pos = 0; line[pos]; pos++) {
 		if (line[pos] == '\t' && tabsize > 0)
 			size += tabsize - (size % tabsize);
 		else