check_everything_connected(): libify

Extract the helper function and the type definition of the iterator
function it uses out of builtin/fetch.c into a separate source and a
header file.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

connected.c

@@ -0,0 +1,62 @@
+#include "cache.h"
+#include "run-command.h"
+#include "sigchain.h"
+#include "connected.h"
+
+/*
+ * If we feed all the commits we want to verify to this command
+ *
+ *  $ git rev-list --verify-objects --stdin --not --all
+ *
+ * and if it does not error out, that means everything reachable from
+ * these commits locally exists and is connected to some of our
+ * existing refs.
+ *
+ * Returns 0 if everything is connected, non-zero otherwise.
+ */
+int check_everything_connected(sha1_iterate_fn fn, int quiet, void *cb_data)
+{
+	struct child_process rev_list;
+	const char *argv[] = {"rev-list", "--verify-objects",
+			      "--stdin", "--not", "--all", NULL, NULL};
+	char commit[41];
+	unsigned char sha1[20];
+	int err = 0;
+
+	if (fn(cb_data, sha1))
+		return err;
+
+	if (quiet)
+		argv[5] = "--quiet";
+
+	memset(&rev_list, 0, sizeof(rev_list));
+	rev_list.argv = argv;
+	rev_list.git_cmd = 1;
+	rev_list.in = -1;
+	rev_list.no_stdout = 1;
+	rev_list.no_stderr = quiet;
+	if (start_command(&rev_list))
+		return error(_("Could not run 'git rev-list'"));
+
+	sigchain_push(SIGPIPE, SIG_IGN);
+
+	commit[40] = '\n';
+	do {
+		memcpy(commit, sha1_to_hex(sha1), 40);
+		if (write_in_full(rev_list.in, commit, 41) < 0) {
+			if (errno != EPIPE && errno != EINVAL)
+				error(_("failed write to rev-list: %s"),
+				      strerror(errno));
+			err = -1;
+			break;
+		}
+	} while (!fn(cb_data, sha1));
+
+	if (close(rev_list.in)) {
+		error(_("failed to close rev-list's stdin: %s"), strerror(errno));
+		err = -1;
+	}
+
+	sigchain_pop(SIGPIPE);
+	return finish_command(&rev_list) || err;
+}