Test some additional exotic rebase setup behavior: that we are
able to set up properly when already in a detached HEAD state,
that the caller specifies all of branch, upstream and onto,
and that the caller specifies branch, upstream and onto by ID.
When performing an in-memory rebase, keep a single index for the
duration, so that callers have the expected index lifecycle and
do not hold on to an index that is free'd out from under them.
Introduce the ability to rebase in-memory or in a bare repository.
When `rebase_options.inmemory` is specified, the resultant `git_rebase`
session will not be persisted to disk. Callers may still analyze
the rebase operations, resolve any conflicts against the in-memory
index and create the commits. Neither `HEAD` nor the working
directory will be updated during this process.
In `git_rebase_operation_current()`, indicate when a rebase has not
started (with `GIT_REBASE_NO_OPERATION`) rather than conflating that
with the first operation being in-progress.
We want to use the "checkout: moving from ..." message in order to let
git know when a change of branch has happened. Make the convenience
functions for this goal write this message.
The signature for the reflog is not something which changes
dynamically. Almost all uses will be NULL, since we want for the
repository's default identity to be used, making it noise.
In order to allow for changing the identity, we instead provide
git_repository_set_ident() and git_repository_ident() which allow a user
to override the choice of signature.
Already cherry-picked commits should not be re-included. If all changes
included in a commit exist in the upstream, then we should error with
GIT_EAPPLIED.
`git_rebase_next` will apply the next patch (or cherry-pick)
operation, leaving the results checked out in the index / working
directory so that consumers can resolve any conflicts, as appropriate.
