Add merge-subtree back

An earlier commit e1b3a2c (Build-in merge-recursive) made the
subtree merge strategy backend unavailable.  This resurrects
it.

A new test t6029 currently only tests the strategy is available,
but it should be enhanced to check the real "subtree" case.

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

Makefile

@@ -261,17 +261,18 @@ PROGRAMS = \
 # Empty...
 EXTRA_PROGRAMS =
 
+# List built-in command $C whose implementation cmd_$C() is not in
+# builtin-$C.o but is linked in as part of some other command.
 BUILT_INS = \
 	git-format-patch$X git-show$X git-whatchanged$X git-cherry$X \
 	git-get-tar-commit-id$X git-init$X git-repo-config$X \
 	git-fsck-objects$X git-cherry-pick$X git-peek-remote$X git-status$X \
+	git-merge-subtree$X \
 	$(patsubst builtin-%.o,git-%$X,$(BUILTIN_OBJS))
 
 # what 'all' will build and 'install' will install, in gitexecdir
 ALL_PROGRAMS = $(PROGRAMS) $(SCRIPTS)
 
-ALL_PROGRAMS += git-merge-subtree$X
-
 # what 'all' will build but not install in gitexecdir
 OTHER_PROGRAMS = git$X gitweb/gitweb.cgi
 
@@ -807,9 +808,6 @@ help.o: help.c common-cmds.h GIT-CFLAGS
 		'-DGIT_MAN_PATH="$(mandir_SQ)"' \
 		'-DGIT_INFO_PATH="$(infodir_SQ)"' $<
 
-git-merge-subtree$X: git-merge-recursive$X
-	$(QUIET_BUILT_IN)$(RM) $@ && ln git-merge-recursive$X $@
-
 $(BUILT_INS): git$X
 	$(QUIET_BUILT_IN)$(RM) $@ && ln git$X $@
 

git.c

@@ -332,6 +332,7 @@ static void handle_internal_command(int argc, const char **argv)
 		{ "merge-file", cmd_merge_file },
 		{ "merge-ours", cmd_merge_ours, RUN_SETUP },
 		{ "merge-recursive", cmd_merge_recursive, RUN_SETUP | NEED_WORK_TREE },
+		{ "merge-subtree", cmd_merge_recursive, RUN_SETUP | NEED_WORK_TREE },
 		{ "mv", cmd_mv, RUN_SETUP | NEED_WORK_TREE },
 		{ "name-rev", cmd_name_rev, RUN_SETUP },
 		{ "pack-objects", cmd_pack_objects, RUN_SETUP },

t/t6029-merge-subtree.sh

@@ -0,0 +1,32 @@
+#!/bin/sh
+
+test_description='subtree merge strategy'
+
+. ./test-lib.sh
+
+test_expect_success setup '
+
+	s="1 2 3 4 5 6 7 8"
+	for i in $s; do echo $i; done >hello &&
+	git add hello &&
+	git commit -m initial &&
+	git checkout -b side &&
+	echo >>hello world &&
+	git add hello &&
+	git commit -m second &&
+	git checkout master &&
+	for i in mundo $s; do echo $i; done >hello &&
+	git add hello &&
+	git commit -m master
+
+'
+
+test_expect_success 'subtree available and works like recursive' '
+
+	git merge -s subtree side &&
+	for i in mundo $s world; do echo $i; done >expect &&
+	diff -u expect hello
+
+'
+
+test_done