http: use off_t to store partial file size

When we try to resume transfer of a partially-downloaded
object or pack, we fopen() the existing file for append,
then use ftell() to get the current position. We use a
"long", which can hold only 2GB on a 32-bit system, even
though packfiles may be larger than that.

Let's switch to using off_t, which should hold any file size
our system is capable of storing. We need to use ftello() to
get the off_t. This is in POSIX and hopefully available
everywhere; if not, we should be able to wrap it by falling
back to ftell(), which would presumably return "-1" on such
a large file (and we would simply skip resuming in that case).

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

http.c

diff --git a/http.c b/http.c
index 5f36846..42f29ce 100644
--- a/http.c
+++ b/http.c
@@ -1205,7 +1205,7 @@
 		curl_easy_setopt(slot->curl, CURLOPT_FILE, result);
 
 		if (target == HTTP_REQUEST_FILE) {
-			long posn = ftell(result);
+			off_t posn = ftello(result);
 			curl_easy_setopt(slot->curl, CURLOPT_WRITEFUNCTION,
 					 fwrite);
 			if (posn > 0)
@@ -1581,7 +1581,7 @@
 struct http_pack_request *new_http_pack_request(
 	struct packed_git *target, const char *base_url)
 {
-	long prev_posn = 0;
+	off_t prev_posn = 0;
 	struct strbuf buf = STRBUF_INIT;
 	struct http_pack_request *preq;
 
@@ -1613,7 +1613,7 @@
 	 * If there is data present from a previous transfer attempt,
 	 * resume where it left off
 	 */
-	prev_posn = ftell(preq->packfile);
+	prev_posn = ftello(preq->packfile);
 	if (prev_posn>0) {
 		if (http_is_verbose)
 			fprintf(stderr,
@@ -1668,7 +1668,7 @@
 	int prevlocal;
 	char prev_buf[PREV_BUF_SIZE];
 	ssize_t prev_read = 0;
-	long prev_posn = 0;
+	off_t prev_posn = 0;
 	struct http_object_request *freq;
 
 	freq = xcalloc(1, sizeof(*freq));