Support line anchors for commit URLs

Anchor format is:

   #diff-<MD5 of path>[[L|R]<start line number>[-[L|R]<end line number>]]

With L line number is relative to the '---' side, and R to the '+++'
side of the diff.  Path of the '+++' side used when it exists, otherwise
path of the '---' side (for removals).

Closes tpope#36

autoload/rhubarb.vim

@@ -268,7 +268,27 @@ function! rhubarb#FugitiveUrl(opts, ...) abort
   else
     let commit = a:opts.commit
   endif
-  if get(a:opts, 'type', '') ==# 'tree' || a:opts.path =~# '/$'
+  if get(a:opts, 'type', '') ==# 'commit'
+    let url = root . '/commit/' . commit
+    let diff = get(a:opts, 'diff', {})
+    if len(diff) > 0 && executable('md5')
+      let url .= '#diff-' . system('md5 -qs ' . s:shellesc(len(diff.newpath) > 0 ? diff.newpath : diff.oldpath))[0:-2]
+      if len(get(diff, 'start', '')) > 0 
+        if diff.start == '+'
+          let url .= 'R' . diff.newstart
+        else
+          let url .= 'L' . diff.oldstart
+        endif
+        if diff.oldend - diff.oldstart + diff.newend - diff.newstart > 0
+          if diff.end == '+'
+            let url .= '-R' . diff.newend
+          else
+            let url .= '-L' . diff.oldend
+          endif
+        endif
+      endif
+    endif
+  elseif get(a:opts, 'type', '') ==# 'tree' || a:opts.path =~# '/$'
     let url = substitute(root . '/tree/' . commit . '/' . path, '/$', '', 'g')
   elseif get(a:opts, 'type', '') ==# 'blob' || a:opts.path =~# '[^/]$'
     let escaped_commit = substitute(commit, '#', '%23', 'g')