config.c: avoid integer truncation in `copy_or_rename_section_in_file()`

There are a couple of spots within `copy_or_rename_section_in_file()`
that incorrectly use an `int` to track an offset within a string, which
may truncate or wrap around to a negative value.

Historically it was impossible to have a line longer than 1024 bytes
anyway, since we used fgets() with a fixed-size buffer of exactly that
length. But the recent change to use a strbuf permits us to read lines
of arbitrary length, so it's possible for a malicious input to cause us
to overflow past INT_MAX and do an out-of-bounds array read.

Practically speaking, however, this should never happen, since it
requires 2GB section names or values, which are unrealistic in
non-malicious circumstances.

Co-authored-by: Jeff King <peff@peff.net>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Taylor Blau <me@ttaylorr.com>

config.c

@@ -3027,9 +3027,10 @@ void git_config_set_multivar(const char *key, const char *value,
 					flags);
 }
 
-static int section_name_match (const char *buf, const char *name)
+static size_t section_name_match (const char *buf, const char *name)
 {
-	int i = 0, j = 0, dot = 0;
+	size_t i = 0, j = 0;
+	int dot = 0;
 	if (buf[i] != '[')
 		return 0;
 	for (i = 1; buf[i] && buf[i] != ']'; i++) {
@@ -3133,15 +3134,14 @@ static int git_config_copy_or_rename_section_in_file(const char *config_filename
 	}
 
 	while (!strbuf_getwholeline(&buf, config_file, '\n')) {
-		unsigned i;
+		size_t i, length;
-		int length;
 		int is_section = 0;
 		char *output = buf.buf;
 		for (i = 0; buf.buf[i] && isspace(buf.buf[i]); i++)
 			; /* do nothing */
 		if (buf.buf[i] == '[') {
 			/* it's a section */
-			int offset;
+			size_t offset;
 			is_section = 1;
 
 			/*