[PATCH v3 7/7] merge: do not exit restore_state() prematurely

From: Elijah Newren <newren@gmail.com>

Previously, if the user:

* Had no local changes before starting the merge
* A merge strategy makes changes to the working tree/index but returns
  with exit status 2

Then we'd call restore_state() to clean up the changes and either let the next merge strategy run (if there is one), or exit telling the user that no merge strategy could handle the merge. Unfortunately, restore_state() did not clean up the changes as expected; that function was a no-op if the stash was a null, and the stash would be null if there were no local changes before starting the merge. So, instead of "Rewinding the tree to pristine..." as the code claimed, restore_state() would leave garbage around in the index and working tree (possibly including conflicts) for either the next merge strategy or for the user after aborting the merge. And in the case of aborting the merge, the user would be unable to run "git merge --abort" to get rid of the unintended leftover conflicts, because the merge control files were not written as it was presumed that we had restored to a clean state already.

Fix the main problem by making sure that restore_state() only skips the stash application if the stash is null rather than skipping the whole function.

However, there is a secondary problem -- since merge.c forks subprocesses to do the cleanup, the in-memory index is left out-of-sync. While there was a refresh_cache(REFRESH_QUIET) call that attempted to correct that, that function would not handle cases where the previous merge strategy added conflicted entries. We need to drop the index and re-read it to handle such cases.

(Alternatively, we could stop forking subprocesses and instead call some appropriate function to do the work which would update the in-memory index automatically. For now, just do the simple fix.)

Also, add a testcase checking this, one for which the octopus strategy fails on the first commit it attempts to merge, and thus which it cannot handle at all and must completely bail on (as per the "exit 2" code path of commit 98efc8f3d8 ("octopus: allow manual resolve on the last round.", 2006-01-13)).

Reported-by: ZheNing Hu <adlternative@gmail.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
---
 builtin/merge.c        | 10 ++++++----
 t/t7607-merge-state.sh | 32 ++++++++++++++++++++++++++++++++
 2 files changed, 38 insertions(+), 4 deletions(-)
 create mode 100755 t/t7607-merge-state.sh