don't error out early, if git not exists

Even if git is not available, one can still use plug.vim
for managing the runtime path.
This allows, to share the same config with several vim instances, some
of which, might not have vim installed (or not have it in $PATH). The
commands `:Plug` and `:PlugClean` seem to be safe even in this case.

Unfortunately in that case, plug.vim errors out on every start.

So let's just move the check for the existance of git to the place,
where it is actually needed.

In theory, we can also just move the check `:if excutable('git')` around
the definition of the all commands that need git, so they would be only
defined on systems that have git installed. But I actually like the way,
how the commands will always be defined, but they error out once one
tries to execute them.

plug.vim

@@ -112,14 +112,17 @@ function! plug#begin(...)
   return 1
 endfunction
 
-function! s:define_commands()
-  command! -nargs=+ -bar Plug call s:add(<args>)
+function! s:git_exists()
   if !executable('git')
     return s:err('`git` executable not found. vim-plug requires git.')
   endif
+endfunction
+
+function! s:define_commands()
+  command! -nargs=+ -bar Plug call s:add(<args>)
+  command! -nargs=0 -bar -bang PlugClean call s:clean(<bang>0)
   command! -nargs=* -bar -bang -complete=customlist,s:names PlugInstall call s:install(<bang>0, [<f-args>])
   command! -nargs=* -bar -bang -complete=customlist,s:names PlugUpdate  call s:update(<bang>0, [<f-args>])
-  command! -nargs=0 -bar -bang PlugClean call s:clean(<bang>0)
   command! -nargs=0 -bar PlugUpgrade if s:upgrade() | execute 'source' s:esc(s:me) | endif
   command! -nargs=0 -bar PlugStatus  call s:status()
   command! -nargs=0 -bar PlugDiff    call s:diff()
@@ -776,6 +779,10 @@ function! s:names(...)
 endfunction
 
 function! s:update_impl(pull, force, args) abort
+  if !s:git_exists()
+    return
+  endif
+
   let args = copy(a:args)
   let threads = (len(args) > 0 && args[-1] =~ '^[1-9][0-9]*$') ?
                   \ remove(args, -1) : get(g:, 'plug_threads', 16)
@@ -1847,6 +1854,10 @@ function! s:clean(force)
 endfunction
 
 function! s:upgrade()
+  if !s:git_exists()
+    return
+  endif
+
   echo 'Downloading the latest version of vim-plug'
   redraw
   let tmp = tempname()
@@ -1880,6 +1891,9 @@ function! s:upgrade_specs()
 endfunction
 
 function! s:status()
+  if !s:git_exists()
+    return
+  endif
   call s:prepare()
   call append(0, 'Checking plugins')
   call append(1, '')
@@ -2012,6 +2026,9 @@ function! s:append_ul(lnum, text)
 endfunction
 
 function! s:diff()
+  if !s:git_exists()
+    return
+  endif
   call s:prepare()
   call append(0, ['Collecting changes ...', ''])
   let cnts = [0, 0]
@@ -2071,6 +2088,9 @@ function! s:revert()
 endfunction
 
 function! s:snapshot(force, ...) abort
+  if !s:git_exists()
+    return
+  endif
   call s:prepare()
   setf vim
   call append(0, ['" Generated by vim-plug',
@@ -2117,4 +2137,3 @@ endif
 
 let &cpo = s:cpo_save
 unlet s:cpo_save
-