previously, in `gitStackedRebase`, there were 3 invocations of
`applyIfNeedsToApply`. there are still 3, but what differs is:

previously, the 3rd would be invoked if and only if the
`config.autoApplyIfNeeded` was true, i.e., it was NOT mandatory.

however, the 1st invocation, which also is NOT mandatory, did not have
such guardrail, and, if the `config.autoApplyIfNeeded` was false, would
prompt the user with the common message:
`need to --apply before continuing. proceed? [Y/n/(a)lways] `

this is wrong, because this (1st) invocation is not mandatory, meanwhile
the message makes it look like it is, thus confusing the user &
their longer term understanding of when an --apply is actually needed.

to further add to the confusion, the 1st invocation worked correctly
apart from the message - it (1st invoc) was not made mandatory, i.e.
would not stop the program execution if user did not allow.

---

now, there's a clear distinction of mandatory vs not,
and `applyIfNeedsToApply` works accordingly to it.
related parts were also cleaned up.

also, created a test for this -- yay!

Signed-off-by: Kipras Melnikovas <[email protected]>