Use custom git branch name for vim-airline

Airline relies on fugitive to get the branch name, but fugitive is
conditionally loaded. So the branch name is not shown. We need to
provide our own function to get the branch name.

autoload/utils.vim

@@ -159,3 +159,12 @@ function! utils#Inside_git_repo() abort
     return v:true
   endif
 endfunction
+
+function! utils#GetGitBranch()
+  let l:res = systemlist('git rev-parse --abbrev-ref HEAD')[0]
+  if match(l:res, 'fatal') != -1
+    return ''
+  else
+    return l:res
+  endif
+endfunction

core/plugins.vim

@@ -421,6 +421,9 @@ let g:airline_symbols.branch = '⎇'
 let g:airline_symbols.paste = 'ρ'
 let g:airline_symbols.spell = 'Ꞩ'
 
+" Custom branch name
+let g:airline#extensions#branch#custom_head = 'utils#GetGitBranch'
+
 " Only show git hunks which are non-zero
 let g:airline#extensions#hunks#non_zero_only = 1