rebase: start implementing it as a builtin

This commit imitates the strategy that was used to convert the
difftool to a builtin. We start by renaming the shell script
`git-rebase.sh` to `git-legacy-rebase.sh` and introduce a
`builtin/rebase.c` that simply executes the shell script version,
unless the config setting `rebase.useBuiltin` is set to `true`.

The motivation behind this is to rewrite all the functionality of the
shell script version in the aforementioned `rebase.c`, one by one and
be able to conveniently test new features by configuring
`rebase.useBuiltin`.

In the original difftool conversion, if sane_execvp() that attempts to
run the legacy scripted version returned with non-negative status, the
command silently exited without doing anything with success, but
sane_execvp() should not return with non-negative status in the first
place, so we use die() to notice such an abnormal case.

We intentionally avoid reading the config directly to avoid
messing up the GIT_* environment variables when we need to fall back to
exec()ing the shell script. The test of builtin rebase can be done by
`git -c rebase.useBuiltin=true rebase ...`

Signed-off-by: Pratik Karki <predatoramigo@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git.c

@@ -521,6 +521,12 @@ static struct cmd_struct commands[] = {
 	{ "pull", cmd_pull, RUN_SETUP | NEED_WORK_TREE },
 	{ "push", cmd_push, RUN_SETUP },
 	{ "read-tree", cmd_read_tree, RUN_SETUP | SUPPORT_SUPER_PREFIX},
+	/*
+	 * NEEDSWORK: Until the rebase is independent and needs no redirection
+	 * to rebase shell script this is kept as is, then should be changed to
+	 * RUN_SETUP | NEED_WORK_TREE
+	 */
+	{ "rebase", cmd_rebase },
 	{ "rebase--helper", cmd_rebase__helper, RUN_SETUP | NEED_WORK_TREE },
 	{ "receive-pack", cmd_receive_pack },
 	{ "reflog", cmd_reflog, RUN_SETUP },