reflog: implement subcommand to drop reflogs

reflog: implement subcommand to drop reflogs

While 'git-reflog(1)' currently allows users to expire reflogs and
delete individual entries, it lacks functionality to completely remove
reflogs for specific references. This becomes problematic in
repositories where reflogs are not needed but continue to accumulate
entries despite setting 'core.logAllRefUpdates=false'.

Add a new 'drop' subcommand to git-reflog that allows users to delete
the entire reflog for a specified reference. Include an '--all' flag to
enable dropping all reflogs from all worktrees and an addon flag
'--single-worktree', to only drop all reflogs from the current worktree.

While here, remove an extraneous newline in the file.

Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>