Move try_merge_command and checkout_fast_forward to libgit.a

These functions are called in sequencer.c, which is part of
libgit.a. This makes libgit.a potentially require builtin/merge.c for
external git commands.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Jeff King <peff@peff.net>

merge.c

@@ -0,0 +1,112 @@
+#include "cache.h"
+#include "commit.h"
+#include "run-command.h"
+#include "resolve-undo.h"
+#include "tree-walk.h"
+#include "unpack-trees.h"
+#include "dir.h"
+
+static const char *merge_argument(struct commit *commit)
+{
+	if (commit)
+		return sha1_to_hex(commit->object.sha1);
+	else
+		return EMPTY_TREE_SHA1_HEX;
+}
+
+int try_merge_command(const char *strategy, size_t xopts_nr,
+		      const char **xopts, struct commit_list *common,
+		      const char *head_arg, struct commit_list *remotes)
+{
+	const char **args;
+	int i = 0, x = 0, ret;
+	struct commit_list *j;
+	struct strbuf buf = STRBUF_INIT;
+
+	args = xmalloc((4 + xopts_nr + commit_list_count(common) +
+			commit_list_count(remotes)) * sizeof(char *));
+	strbuf_addf(&buf, "merge-%s", strategy);
+	args[i++] = buf.buf;
+	for (x = 0; x < xopts_nr; x++) {
+		char *s = xmalloc(strlen(xopts[x])+2+1);
+		strcpy(s, "--");
+		strcpy(s+2, xopts[x]);
+		args[i++] = s;
+	}
+	for (j = common; j; j = j->next)
+		args[i++] = xstrdup(merge_argument(j->item));
+	args[i++] = "--";
+	args[i++] = head_arg;
+	for (j = remotes; j; j = j->next)
+		args[i++] = xstrdup(merge_argument(j->item));
+	args[i] = NULL;
+	ret = run_command_v_opt(args, RUN_GIT_CMD);
+	strbuf_release(&buf);
+	i = 1;
+	for (x = 0; x < xopts_nr; x++)
+		free((void *)args[i++]);
+	for (j = common; j; j = j->next)
+		free((void *)args[i++]);
+	i += 2;
+	for (j = remotes; j; j = j->next)
+		free((void *)args[i++]);
+	free(args);
+	discard_cache();
+	if (read_cache() < 0)
+		die(_("failed to read the cache"));
+	resolve_undo_clear();
+
+	return ret;
+}
+
+int checkout_fast_forward(const unsigned char *head,
+			  const unsigned char *remote,
+			  int overwrite_ignore)
+{
+	struct tree *trees[MAX_UNPACK_TREES];
+	struct unpack_trees_options opts;
+	struct tree_desc t[MAX_UNPACK_TREES];
+	int i, fd, nr_trees = 0;
+	struct dir_struct dir;
+	struct lock_file *lock_file = xcalloc(1, sizeof(struct lock_file));
+
+	refresh_cache(REFRESH_QUIET);
+
+	fd = hold_locked_index(lock_file, 1);
+
+	memset(&trees, 0, sizeof(trees));
+	memset(&opts, 0, sizeof(opts));
+	memset(&t, 0, sizeof(t));
+	if (overwrite_ignore) {
+		memset(&dir, 0, sizeof(dir));
+		dir.flags |= DIR_SHOW_IGNORED;
+		setup_standard_excludes(&dir);
+		opts.dir = &dir;
+	}
+
+	opts.head_idx = 1;
+	opts.src_index = &the_index;
+	opts.dst_index = &the_index;
+	opts.update = 1;
+	opts.verbose_update = 1;
+	opts.merge = 1;
+	opts.fn = twoway_merge;
+	setup_unpack_trees_porcelain(&opts, "merge");
+
+	trees[nr_trees] = parse_tree_indirect(head);
+	if (!trees[nr_trees++])
+		return -1;
+	trees[nr_trees] = parse_tree_indirect(remote);
+	if (!trees[nr_trees++])
+		return -1;
+	for (i = 0; i < nr_trees; i++) {
+		parse_tree(trees[i]);
+		init_tree_desc(t+i, trees[i]->buffer, trees[i]->size);
+	}
+	if (unpack_trees(nr_trees, t, &opts))
+		return -1;
+	if (write_cache(fd, active_cache, active_nr) ||
+		commit_locked_index(lock_file))
+		die(_("unable to write new index file"));
+	return 0;
+}