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>

1 files changed, 1 insertions, 0 deletions

diff --git a/.gitignore b/.gitignore
index 3284a1e9b1..ec23959014 100644
--- a/.gitignore
+++ b/.gitignore
@@ -78,6 +78,7 @@
 /git-init-db
 /git-interpret-trailers
 /git-instaweb
+/git-legacy-rebase
 /git-log
 /git-ls-files
 /git-ls-remote