Status flags of .git repository are shown next to the branch name as
below.
  [master*+$%]

Try to mimic '$(__git_ps1 " (%s)")' of 'git-completion.bash'
(GIT_PS1_SHOWDIRTYSTATE / GIT_PS1_SHOWSTASHSTATE / GIT_PS1_SHOWUNTRACKEDFILES)

  # dirty_flag - '*': modified, but not staged flag
                 '+': staged (to be committed) flag
  # stash_flag - '$': something is stashed
  # untracked  - '%': there're untracked files

In vim, checking repo_status is done by calling 'git' externally,
vim-user can be tormented with severe performance degradations.
Accordingly, makes status checking as less as possible - only when
.git/index is modified, and files in .git work tree are modified.

To enable this feature, declare below variables somewhere in vimrc

    let g:fugitive_stl_showdirtystate = 1
    let g:fugitive_stl_showstashstate = 1
    let g:fugitive_stl_showuntrackedfiles = 1

Several variables which contain the repo status are combined into single
Dictionary.

Also, git_index_mod_time_old gets changed its type into buffer(b:) due
to support the situation when multiple 'index' files exist in one work
tree, such as submodule.