string-list API: change "nr" and "alloc" to "size_t"

Change the "nr" and "alloc" members of "struct string_list" to use
"size_t" instead of "nr". On some platforms the size of an "unsigned
int" will be smaller than a "size_t", e.g. a 32 bit unsigned v.s. 64
bit unsigned. As "struct string_list" is a generic API we use in a lot
of places this might cause overflows.

As one example: code in "refs.c" keeps track of the number of refs
with a "size_t", and auxiliary code in builtin/remote.c in
get_ref_states() appends those to a "struct string_list".

While we're at it split the "nr" and "alloc" in string-list.h across
two lines, which is the case for most such struct member
declarations (e.g. in "strbuf.h" and "strvec.h").

Changing e.g. "int i" to "size_t i" in run_and_feed_hook() isn't
strictly necessary, and there are a lot more cases where we'll use a
local "int", "unsigned int" etc. variable derived from the "nr" in the
"struct string_list". But in that case as well as
add_wrapped_shortlog_msg() in builtin/shortlog.c we need to adjust the
printf format referring to "nr" anyway, so let's also change the other
variables referring to it.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit-graph.c

@@ -1690,10 +1690,10 @@ static int fill_oids_from_packs(struct write_commit_graph_context *ctx,
 	dirlen = packname.len;
 	if (ctx->report_progress) {
 		strbuf_addf(&progress_title,
-			    Q_("Finding commits for commit graph in %d pack",
-			       "Finding commits for commit graph in %d packs",
+			    Q_("Finding commits for commit graph in %"PRIuMAX" pack",
+			       "Finding commits for commit graph in %"PRIuMAX" packs",
 			       pack_indexes->nr),
-			    pack_indexes->nr);
+			    (uintmax_t)pack_indexes->nr);
 		ctx->progress = start_delayed_progress(progress_title.buf, 0);
 		ctx->progress_done = 0;
 	}