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>