The function `merge_diff_mark_similarity_exact` may error our early and,
when it does so, free the `ours_deletes_by_oid` and
`theirs_deletes_by_oid` variables. While the first one can never be
uninitialized due to the first call actually assigning to it, the second
variable can be freed without being initialized.

Fix the issue by initializing both variables to `NULL`.

The current exact rename detection has order n^2 complexity.
We can do better by using a map to first aggregate deletes and
using that to match deletes to adds.

This results in a substantial performance improvement for merges
with a large quantity of adds and deletes.

The `path_repository` variable is actually confusing to think
about, as it is not always clear what the repository actually is.
It may either be the path to the folder containing worktree and
.git directory, the path to .git itself, a worktree or something
entirely different. Actually, the intent of the variable is to
hold the path to the gitdir, which is either the .git directory
or the bare repository.

Rename the variable to `gitdir` to avoid confusion. While at it,
also rename `path_gitlink` to `gitlink` to improve consistency.

merge: set default rename threshold

When `GIT_MERGE_FIND_RENAMES` is set, provide a default for
`rename_threshold` when it is unset.

Error messages should be sentence fragments, and therefore:

1. Should not begin with a capital letter,
2. Should not conclude with punctuation, and
3. Should not end a sentence and begin a new one