revision.h: store hidden refs in a strvec

In subsequent commits, it will be convenient to have a 'const char **'
of hidden refs (matching `transfer.hiderefs`, `uploadpack.hideRefs`,
etc.), instead of a `string_list`.

Convert spots throughout the tree that store the list of hidden refs
from a `string_list` to a `strvec`.

Note that in `parse_hide_refs_config()` there is an ugly const-cast used
to avoid an extra copy of each value before trimming any trailing slash
characters. This could instead be written as:

    ref = xstrdup(value);
    len = strlen(ref);
    while (len && ref[len - 1] == '/')
            ref[--len] = '\0';
    strvec_push(hide_refs, ref);
    free(ref);

but the double-copy (once when calling `xstrdup()`, and another via
`strvec_push()`) is wasteful.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

ls-refs.c

@@ -72,7 +72,7 @@ struct ls_refs_data {
 	unsigned symrefs;
 	struct strvec prefixes;
 	struct strbuf buf;
-	struct string_list hidden_refs;
+	struct strvec hidden_refs;
 	unsigned unborn : 1;
 };
 
@@ -155,7 +155,7 @@ int ls_refs(struct repository *r, struct packet_reader *request)
 	memset(&data, 0, sizeof(data));
 	strvec_init(&data.prefixes);
 	strbuf_init(&data.buf, 0);
-	string_list_init_dup(&data.hidden_refs);
+	strvec_init(&data.hidden_refs);
 
 	git_config(ls_refs_config, &data);
 
@@ -197,7 +197,7 @@ int ls_refs(struct repository *r, struct packet_reader *request)
 	packet_fflush(stdout);
 	strvec_clear(&data.prefixes);
 	strbuf_release(&data.buf);
-	string_list_clear(&data.hidden_refs, 0);
+	strvec_clear(&data.hidden_refs);
 	return 0;
 }