rebase: move `reset_head()` into a better spot

rebase: move `reset_head()` into a better spot

Over the next commits, we want to make use of it in `run_am()` (i.e.
running the `--am` backend directly, without detouring to Unix shell
script code) which in turn will be called from `run_specific_rebase()`.

So let's move it before that latter function.

This commit is best viewed using --color-moved.

Signed-off-by: Johannes Schindelin <redacted>
Signed-off-by: Junio C Hamano <redacted>