[add] short_path formatter in section_c

[update] test

autoload/airline/formatter/short_path.vim

@@ -0,0 +1,8 @@
+scriptencoding utf-8
+
+function! airline#formatter#short_path#format(val) abort
+  if get(g:, 'airline_stl_path_style', 'default') ==# 'short'
+    return '%{pathshorten(expand("'.a:val.'"))}%'
+  endif
+  return a:val
+endfunction

autoload/airline/init.vim

@@ -146,7 +146,7 @@ function! airline#init#bootstrap()
         \ 'function': 'airline#parts#readonly',
         \ 'accent': 'red',
         \ })
-  call airline#parts#define_raw('file', '%f%m')
+  call airline#parts#define_raw('file', airline#formatter#short_path#format('%f%m'))
   call airline#parts#define_raw('path', '%F%m')
   call airline#parts#define('linenr', {
         \ 'raw': '%{g:airline_symbols.linenr}%l',

t/init.vim

@@ -33,6 +33,11 @@ describe 'init sections'
     Expect g:airline_section_c == '%<%f%m %#__accent_red#%{airline#util#wrap(airline#parts#readonly(),0)}%#__restore__#%#__accent_bold#%#__restore__#'
   end
 
+  it 'section c should be file and coc_status short style'
+    let g:airline_stl_path_style = 'short'
+    Expect g:airline_section_c == '%<%{airline#formatter#short_path#format("%f%m")}% %#__accent_red#%{airline#util#wrap(airline#parts#readonly(),0)}%#__restore__#%#__accent_bold#%#__restore__#'
+  end
+
   it 'section x should be filetype'
     Expect g:airline_section_x == '%{airline#util#prepend("",0)}%{airline#util#prepend("",0)}%{airline#util#prepend("",0)}%{airline#util#prepend("",0)}%{airline#util#wrap(airline#parts#filetype(),0)}'
   end