blame: widen find_line_starts() len parameter to size_t

Prep for the upcoming blame_scoreboard.final_buf_size widening:
prepare_lines() will pass a size_t through to find_line_starts(),
and the other caller (fill_origin_blob() via o->file.size) already
goes through long->size_t promotion. The function is file-static
and only uses len as a loop bound.

Assisted-by: Opus 4.7
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

blame.c

@@ -335,7 +335,7 @@ static const char *get_next_line(const char *start, const char *end)
 }
 
 static int find_line_starts(int **line_starts, const char *buf,
-			    unsigned long len)
+			    size_t len)
 {
 	const char *end = buf + len;
 	const char *p;