Provide :GBrowse behavior in blame buffers

If a line number is given, browse to the commit on that line.
Otherwise, browse to the file, same as if called from the original
buffer.

It would probably make more sense to open the corresponding blame page
on providers that support it, but that will have to wait on an API
change.

References #1214

autoload/fugitive.vim

@@ -5576,7 +5576,10 @@ endfunction
 
 function! s:BlameCommitFileLnum(...) abort
   let line = a:0 ? a:1 : getline('.')
-  let state = a:0 ? a:2 : s:TempState()
+  let state = a:0 > 1 ? a:2 : s:TempState()
+  if get(state, 'filetype', '') !=# 'fugitiveblame'
+    return ['', '', 0]
+  endif
   let commit = matchstr(line, '^\^\=[?*]*\zs\x\+')
   if commit =~# '^0\+$'
     let commit = ''
@@ -6126,9 +6129,6 @@ function! fugitive#BrowseCommand(line1, count, range, bang, mods, arg, args) abo
     else
       let rev = arg
     endif
-    if rev ==# ''
-      let rev = s:DirRev(@%)[1]
-    endif
     if rev =~? '^\a\a\+:[\/][\/]' && rev !~? '^fugitive:'
       let rev = substitute(rev, '\\\@<![#!]\|\\\@<!%\ze\w', '\\&', 'g')
     elseif rev ==# ':'
@@ -6140,7 +6140,17 @@ function! fugitive#BrowseCommand(line1, count, range, bang, mods, arg, args) abo
     endif
     exe s:DirCheck(dir)
     if empty(expanded)
-      let expanded = s:Relative(':(top)', dir)
+      let bufname = s:BufName('%')
+      let expanded = s:DirRev(bufname)[1]
+      if empty(expanded)
+        let expanded = fugitive#Path(bufname, ':(top)', dir)
+      endif
+      if a:count > 0 && bufname !=# bufname('')
+        let blame = s:BlameCommitFileLnum(getline(a:count))
+        if len(blame[0])
+          let expanded = blame[0]
+        endif
+      endif
     endif
     let cdir = FugitiveVimPath(fugitive#CommonDir(dir))
     for subdir in ['tags/', 'heads/', 'remotes/']