merge-ll: rename from ll-merge

merge-ll: rename from ll-merge

A long term (but rather minor) pet-peeve of mine was the name
ll-merge.[ch].  I thought it made it harder to realize what stuff was
related to merging when I was working on the merge machinery and trying
to improve it.

Further, back in d1cbe1e6d8a ("hash-ll.h: split out of hash.h to remove
dependency on repository.h", 2023-04-22), we have split the portions of
hash.h that do not depend upon repository.h into a "hash-ll.h" (due to
the recommendation to use "ll" for "low-level" in its name[1], but which
I used as a suffix precisely because of my distaste for "ll-merge").
When we discussed adding additional "*-ll.h" files, a request was made
that we use "ll" consistently as either a prefix or a suffix.  Since it
is already in use as both a prefix and a suffix, the only way to do so
is to rename some files.

Besides my distaste for the ll-merge.[ch] name, let me also note that
the files
  ll-fsmonitor.h, ll-hash.h, ll-merge.h, ll-object-store.h, ll-read-cache.h
would have essentially nothing to do with each other and make no sense
to group.  But giving them the common "ll-" prefix would group them.  Using
"-ll" as a suffix thus seems just much more logical to me.  Rename
ll-merge.[ch] to merge-ll.[ch] to achieve this consistency, and to
ensure we get a more logical grouping of files.

[1] https://lore.kernel.org/git/kl6lsfcu1g8w.fsf@chooglen-macbookpro.roam.corp.google.com/

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