ref-filter: clarify lstrip/rstrip component counting

When a strip option to the %(refname) placeholder is asked to leave N
path components, we first count up the path components to know how many
to remove. That happens with a loop like this:

	/* Find total no of '/' separated path-components */
	for (i = 0; p[i]; p[i] == '/' ? i++ : *p++)
		;

which is a little hard to understand for two reasons.

First, the dereference in "*p++" is seemingly useless, since nobody
looks at the result. And static analyzers like Coverity will complain
about that. But removing the "*" will cause gcc to complain with
-Wint-conversion, since the two sides of the ternary do not match (one
is a pointer and the other an int).

Second, it is not clear what the meaning of "p" is at each iteration of
the loop, as its position with respect to our walk over the string
depends on how many slashes we've seen. The answer is that by itself, it
doesn't really mean anything: "p + i" represents the current state of
our walk, with "i" counting up slashes, and "p" by itself essentially
meaningless.

None of this behaves incorrectly, but ultimately the loop is just
counting the slashes in the refname. We can do that much more simply
with a for-loop iterating over the string and a separate slash counter.

We can also drop the comment, which is somewhat misleading. We are
counting slashes, not components (and a comment later in the function
makes it clear that we must add one to compensate). In the new code it
is obvious that we are counting slashes here.

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

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