branch: fugitive renamed its functions

this caused that the branch extension was not correctly working anymore
because of a refactoring of fugitive tpope/vim-fugitive@5d11ff7

Solution: Move the existence check for the fugitive plugin into a
separate function and call it from there in all places that check the
fugitive plugin. Do the same for lawrencium and vcscommand check.

fixes #605 #1739

autoload/airline/extensions/branch.vim

@@ -3,19 +3,6 @@
 
 scriptencoding utf-8
 
-function! s:has_fugitive()
-  return exists('*fugitive#head')
-endfunction
-function! s:has_lawrencium()
-  return exists('*lawrencium#statusline')
-endfunction
-function! s:has_vcscommand()
-  return get(g:, 'airline#extensions#branch#use_vcscommand', 0) && exists('*VCSCommandGetStatusLine')
-endfunction
-function! s:has_custom_scm()
-  return !empty(get(g:, 'airline#extensions#branch#custom_head', ''))
-endfunction
-
 " s:vcs_config contains static configuration of VCSes and their status relative
 " to the active file.
 " 'branch'    - The name of currently active branch. This field is empty iff it
@@ -94,7 +81,7 @@ let s:names = {'0': 'index', '1': 'orig', '2':'fetch', '3':'merge'}
 let s:sha1size = get(g:, 'airline#extensions#branch#sha1_len', 7)
 
 function! s:update_git_branch()
-  if !s:has_fugitive()
+  if !airline#util#has_fugitive()
     let s:vcs_config['git'].branch = ''
     return
   endif
@@ -128,7 +115,7 @@ function! s:display_git_branch()
 endfunction
 
 function! s:update_hg_branch()
-  if s:has_lawrencium()
+  if airline#util#has_lawrencium()
     let cmd='LC_ALL=C hg qtop'
     let stl=lawrencium#statusline()
     let file=expand('%:p')
@@ -253,7 +240,7 @@ function! airline#extensions#branch#head()
   endfor
 
   if empty(heads)
-    if s:has_vcscommand()
+    if airline#util#has_vcscommand()
       call VCSCommandEnableBufferSetup()
       if exists('b:VCSCommandBufferInfo')
         let b:airline_head = s:format_name(get(b:VCSCommandBufferInfo, 0, ''))
@@ -262,7 +249,7 @@ function! airline#extensions#branch#head()
   endif
 
   if empty(heads)
-    if s:has_custom_scm()
+    if airline#util#has_custom_scm()
       try
         let Fn = function(g:airline#extensions#branch#custom_head)
         let b:airline_head = Fn()

autoload/airline/extensions/fugitiveline.vim

@@ -3,7 +3,7 @@
 
 scriptencoding utf-8
 
-if !exists('*fugitive#head')
+if !airline#util#has_fugitive()
   finish
 endif