merge-recursive: give notice when submodule commit gets fast-forwarded

Inform the user about an automatically fast-forwarded submodule. The
silent merge behavior was introduced by commit 68d03e4a6e ("Implement
automatic fast-forward merge for submodules", 2010-07-07)).

Signed-off-by: Leif Middelschulte <Leif.Middelschulte@gmail.com>
Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

merge-recursive.c

@@ -1093,10 +1093,26 @@ static int merge_submodule(struct merge_options *o,
 	/* Case #1: a is contained in b or vice versa */
 	if (in_merge_bases(commit_a, commit_b)) {
 		oidcpy(result, b);
+		if (show(o, 3)) {
+			output(o, 3, _("Fast-forwarding submodule %s to the following commit:"), path);
+			output_commit_title(o, commit_b);
+		} else if (show(o, 2))
+			output(o, 2, _("Fast-forwarding submodule %s to %s"), path, oid_to_hex(b));
+		else
+			; /* no output */
+
 		return 1;
 	}
 	if (in_merge_bases(commit_b, commit_a)) {
 		oidcpy(result, a);
+		if (show(o, 3)) {
+			output(o, 3, _("Fast-forwarding submodule %s to the following commit:"), path);
+			output_commit_title(o, commit_a);
+		} else if (show(o, 2))
+			output(o, 2, _("Fast-forwarding submodule %s to %s"), path, oid_to_hex(a));
+		else
+			; /* no output */
+
 		return 1;
 	}