interpret_branch_name: rename "cp" variable to "at"

In the original version of this function, "cp" acted as a
pointer to many different things. Since the refactoring in
the last patch, it only marks the at-sign in the string.
Let's use a more descriptive variable name.

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

sha1_name.c

@@ -1119,7 +1119,7 @@ static int interpret_upstream_mark(const char *name, int namelen,
  */
 int interpret_branch_name(const char *name, int namelen, struct strbuf *buf)
 {
-	char *cp;
+	char *at;
 	int len = interpret_nth_prior_checkout(name, buf);
 
 	if (!namelen)
@@ -1134,15 +1134,15 @@ int interpret_branch_name(const char *name, int namelen, struct strbuf *buf)
 			return reinterpret(name, namelen, len, buf);
 	}
 
-	cp = strchr(name, '@');
-	if (!cp)
+	at = strchr(name, '@');
+	if (!at)
 		return -1;
 
-	len = interpret_empty_at(name, namelen, cp - name, buf);
+	len = interpret_empty_at(name, namelen, at - name, buf);
 	if (len > 0)
 		return reinterpret(name, namelen, len, buf);
 
-	len = interpret_upstream_mark(name, namelen, cp - name, buf);
+	len = interpret_upstream_mark(name, namelen, at - name, buf);
 	if (len > 0)
 		return len;