builtin rebase: support `--allow-empty-message` option
This commit introduces the `--allow-empty-message` option to
`builtin/rebase.c`. The motivation behind this option is: if there are
empty messages (which is not allowed in Git by default, but can be
imported from different version control systems), the rebase will fail.
Using `--allow-empty-message` overrides that behaviour which will allow
the commits having empty messages to continue in rebase operation.
Note: a very recent change made this the default in the shell scripted
`git rebase`, therefore the builtin rebase does the same.
Signed-off-by: Pratik Karki <predatoramigo@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>