provide more errors for the "merge into empty head" case

A squash merge into an unborn branch could be implemented by building the
index from the merged-from branch, and doing a single commit, but this is
not supported yet.

A non-fast-forward merge into an unborn branch does not make any sense,
because you cannot make a merge commit if you don't have a commit to use
as the parent.

Signed-off-by: Paolo Bonzini <bonzini@gnu.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-merge.c

@@ -833,6 +833,11 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 		if (argc != 1)
 			die("Can merge only exactly one commit into "
 				"empty head");
+		if (squash)
+			die("Squash commit into empty head not supported yet");
+		if (!allow_fast_forward)
+			die("Non-fast-forward commit does not make sense into "
+			    "an empty head");
 		remote_head = peel_to_type(argv[0], 0, NULL, OBJ_COMMIT);
 		if (!remote_head)
 			die("%s - not something we can merge", argv[0]);