The diff_populate_filespec() method is used to describe the diff after a
merge operation is complete. In order to avoid expanding a sparse index,
the reuse_worktree_file() needs to be adapted to ignore files that are
outside of the sparse-checkout cone. The file names and OIDs used for
this check come from the merged tree in the case of the ORT strategy,
not the index, hence the ability to look into these paths without having
already expanded the index.

The work done by reuse_worktree_file() is only an optimization, and
requires the file being on disk for it to be of any value. Thus, it is
safe to exit the method early if we do not expect the file on disk.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>

Allow 'git merge' to operate without expanding a sparse index, at least
not immediately. The index still will be expanded in a few cases:

1. If the merge strategy is 'recursive', then we enable
   command_requires_full_index at the start of the merge_recursive()
   method. We expect sparse-index users to also have the 'ort' strategy
   enabled.

2. With the 'ort' strategy, if the merge results in a conflicted file,
   then we expand the index before updating the working tree. The loop
   that iterates over the worktree replaces index entries and tracks
   'origintal_cache_nr' which can become completely wrong if the index
   expands in the middle of the operation. This safety valve is
   important before that loop starts. A later change will focus this
   to only expand if we indeed have a conflict outside of the
   sparse-checkout cone.

3. Other merge strategies are executed as a 'git merge-X' subcommand,
   and those strategies are currently protected with the
   'command_requires_full_index' guard.

Some test updates are required, including a mistaken 'git checkout -b'
that did not specify the base branch, causing merges to be fast-forward
merges.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>

Merge conflicts happen often enough to want to avoid expanding a sparse
index when they happen, as long as those conflicts are within the
sparse-checkout cone. If a conflict exists outside of the
sparse-checkout cone, then we still need to expand before iterating over
the index entries. This is critical to do in advance because of how the
original_cache_nr is tracked to allow inserting and replacing cache
entries.

Iterate over the conflicted files and check if any paths are outside of
the sparse-checkout cone. If so, then expand the full index.

Add a test that demonstrates that we do not expand the index, even when
we hit a conflict within the sparse-checkout cone.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>

Add tests to check that cherry-pick and rebase behave the same in the
sparse-index case as in the full index cases. These tests are agnostic
to GIT_TEST_MERGE_ALGORITHM, so a full CI test suite will check both the
'ort' and 'recursive' strategies on this test.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>

The sequencer is used by 'cherry-pick' and 'rebase' to sequence a list
of operations that modify the index. Since we intend to remove the need
for 'command_requires_full_index', we need to ensure the sparse index is
expanded every time it is written to disk between these steps. That is,
unless the merge strategy is 'ort' where the index can remain sparse
throughout.

There are two main places to be extra careful about a full index:

1. Right before calling merge_trees(), ensure the index is full. This
   happens within an 'else' where the 'if' block checks if the 'ort'
   strategy is selected.

2. During read_and_refresh_cache(), the index might be written to disk
   and converted to sparse in the process. Ensure it expands back to
   full afterwards by checking if the strategy is _not_ 'ort'. This
   'if' statement is the logical negation of the 'if' in item (1).

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>

The hard work was already done with 'git merge' and the ORT strategy.
Just add extra tests to see that we get the expected results in the
non-conflict cases.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>