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>

revision.h

@@ -10,6 +10,7 @@
 #include "decorate.h"
 #include "ident.h"
 #include "list-objects-filter-options.h"
+#include "strvec.h"
 
 /**
  * The revision walking API offers functions to build a list of revisions
@@ -95,7 +96,7 @@ struct ref_exclusions {
 	 * Hidden refs is a list of patterns that is to be hidden via
 	 * `ref_is_hidden()`.
 	 */
-	struct string_list hidden_refs;
+	struct strvec hidden_refs;
 
 	/*
 	 * Indicates whether hidden refs have been configured. This is to
@@ -110,7 +111,7 @@ struct ref_exclusions {
  */
 #define REF_EXCLUSIONS_INIT { \
 	.excluded_refs = STRING_LIST_INIT_DUP, \
-	.hidden_refs = STRING_LIST_INIT_DUP, \
+	.hidden_refs = STRVEC_INIT, \
 }
 
 struct oidset;