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>

Makefile

@@ -614,7 +614,7 @@ SCRIPT_SH += git-merge-one-file.sh
 SCRIPT_SH += git-merge-resolve.sh
 SCRIPT_SH += git-mergetool.sh
 SCRIPT_SH += git-quiltimport.sh
-SCRIPT_SH += git-rebase.sh
+SCRIPT_SH += git-legacy-rebase.sh
 SCRIPT_SH += git-remote-testgit.sh
 SCRIPT_SH += git-request-pull.sh
 SCRIPT_SH += git-stash.sh
@@ -1073,6 +1073,7 @@ BUILTIN_OBJS += builtin/pull.o
 BUILTIN_OBJS += builtin/push.o
 BUILTIN_OBJS += builtin/range-diff.o
 BUILTIN_OBJS += builtin/read-tree.o
+BUILTIN_OBJS += builtin/rebase.o
 BUILTIN_OBJS += builtin/rebase--helper.o
 BUILTIN_OBJS += builtin/receive-pack.o
 BUILTIN_OBJS += builtin/reflog.o