parse_config_key(): return subsection len as size_t

We return the length to a subset of a string using an "int *"
out-parameter. This is fine most of the time, as we'd expect config keys
to be relatively short, but it could behave oddly if we had a gigantic
config key. A more appropriate type is size_t.

Let's switch over, which lets our callers use size_t as appropriate
(they are bound by our type because they must pass the out-parameter as
a pointer). This is mostly just a cleanup to make it clear this code
handles long strings correctly. In practice, our config parser already
chokes on long key names (because of a similar int/size_t mixup!).

When doing an int/size_t conversion, we have to be careful that nobody
was trying to assign a negative value to the variable. I manually
confirmed that for each case here. They tend to just feed the result to
xmemdupz() or similar; in a few cases I adjusted the parameter types for
helper functions to make sure the size_t is preserved.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/reflog.c

@@ -459,7 +459,7 @@ static struct reflog_expire_cfg *find_cfg_ent(const char *pattern, size_t len)
 static int reflog_expire_config(const char *var, const char *value, void *cb)
 {
 	const char *pattern, *key;
-	int pattern_len;
+	size_t pattern_len;
 	timestamp_t expire;
 	int slot;
 	struct reflog_expire_cfg *ent;