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>
Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
a33806398a