Return '' from buffer.path() if outside work tree

In reference to #278.

doc/fugitive.txt

@@ -122,7 +122,7 @@ that are part of Git repositories).
 
 :{range}Gread! [args]  |:read| the output of a Git command after {range}.
 
-                                                *fugitive-:Gwrite*
+                                        *fugitive-:Gw* *fugitive-:Gwrite*
 :Gwrite                 Write to the current file's path and stage the results.
                         When run in a work tree file, it is effectively git
                         add.  Elsewhere, it is effectively git-checkout.  A
@@ -203,7 +203,7 @@ that are part of Git repositories).
                         To use with GitHub FI, point g:fugitive_github_domains
                         at a list of domains:
 >
-                        let g:fugitive_github_domains = ['git.example.com']
+                        let g:fugitive_github_domains = ['https://example.com']
 ~
 :[range]Gbrowse!        Like :Gbrowse, but put the URL on the clipboard rather
                         than opening it.

plugin/fugitive.vim

@@ -506,9 +506,9 @@ function! s:buffer_path(...) dict abort
   let rev = matchstr(self.spec(),'^fugitive://.\{-\}//\zs.*')
   if rev != ''
     let rev = s:sub(rev,'\w*','')
-  elseif self.repo().bare()
+  elseif self.spec()[0 : len(self.repo().dir())] ==# self.repo().dir() . '/'
     let rev = '/.git'.self.spec()[strlen(self.repo().dir()) : -1]
-  else
+  elseif !self.repo().bare() && self.spec()[0 : len(self.repo().tree())] ==# self.repo().tree() . '/'
     let rev = self.spec()[strlen(self.repo().tree()) : -1]
   endif
   return s:sub(s:sub(rev,'.\zs/$',''),'^/',a:0 ? a:1 : '')
@@ -1870,14 +1870,19 @@ endfunction
 function! s:github_url(repo,url,rev,commit,path,type,line1,line2) abort
   let path = a:path
   let domain_pattern = 'github\.com'
-  for domain in exists('g:fugitive_github_domains') ? g:fugitive_github_domains : []
-    let domain_pattern .= '\|' . escape(domain, '.')
+  let domains = exists('g:fugitive_github_domains') ? g:fugitive_github_domains : []
+  for domain in domains
+    let domain_pattern .= '\|' . escape(split(domain, '://')[-1], '.')
   endfor
   let repo = matchstr(a:url,'^\%(https\=://\|git://\|git@\)\zs\('.domain_pattern.'\)[/:].\{-\}\ze\%(\.git\)\=$')
   if repo ==# ''
     return ''
   endif
-  let root = 'https://' . s:sub(repo,':','/')
+  if index(domains, 'http://' . matchstr(repo, '^[^:/]*')) >= 0
+    let root = 'http://' . s:sub(repo,':','/')
+  else
+    let root = 'https://' . s:sub(repo,':','/')
+  endif
   if path =~# '^\.git/refs/heads/'
     let branch = a:repo.git_chomp('config','branch.'.path[16:-1].'.merge')[11:-1]
     if branch ==# ''