help: move git-diff and git-reset to different groups

The third column in command-list.txt determines what group a common command is printed in 'git help'. "git reset" is currently in the "work on the current change (see also: git help everyday)" group. While it's true that "git reset" can manipulate the index and can be in this group, its unique functionality is resetting HEAD, which should be the "grow, mark, tweak history" group. Moving it there will also avoid the confusion because both 'restore' and 'reset' are in the same group, next to each other. While looking at the 'group, mark, tweak history', I realize "git diff" should not be there. All the commands in this group is about _changing_ the commit history while "git diff" is a read-only operation. It fits better in the "examine the history and state" group (especially when "git status", its close friend, is already there). This is what we have after the reorganization: work on the current change (see also: git help everyday) add Add file contents to the index mv Move or rename a file, a directory, or a symlink restore Restore working tree files rm Remove files from the working tree and from the index examine the history and state (see also: git help revisions) bisect Use binary search to find the commit that introduced a bug diff Show changes between commits, commit and working tree, etc grep Print lines matching a pattern log Show commit logs show Show various types of objects status Show the working tree status grow, mark and tweak your common history branch List, create, or delete branches commit Record changes to the repository merge Join two or more development histories together rebase Reapply commits on top of another base tip reset Reset current HEAD to the specified state switch Switch branches tag Create, list, delete or verify a tag object signed with GPG Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>