Extend :Gbrowse to support line anchors for commits

If the first line of :Gbrowse range is inside commit's diff hunk, add
'diff' field to provider metadata with dictionary containing:

  * 'cursor': ' '/'-'/'+' when cursor is on the
  context/removed/added line
  * 'oldpath': path of the '---' side of the diff hunk (empty for
  additions)
  * 'newpath': path of the '+++' side of the diff hunk (empty for
  removals)
  * 'oldline': line number for the cursor position in the '---' side
  * 'newline': line number for the cursor position in the '+++' side

When cursor is on the added or removed line, opposite side line number
is taken as for the *next* context line (even if it doesn't exist).
Exception is completely missing paths (additions or removals) -
corresponding line number is always 0. This is the same behavior as in
the diff hunk header.

autoload/fugitive.vim

@@ -3967,6 +3967,25 @@ function! s:Browse(bang,line1,count,...) abort
           \ 'line1': line1,
           \ 'line2': line2}
 
+    if type ==# 'commit' && a:count
+      let lnum = line('.') - 1
+      let offsets = {' ':0,'+':0,'-':0} 
+      try
+        while getline(lnum) !~# '^@@ -\d\+\%(,\d\+\)\= +\d\+'
+          let offsets[getline(lnum)[0]] += 1
+          let lnum -= 1
+        endwhile
+        let pattern = ' \(\/dev\/null\|[abciow12]\/\)\zs.*'
+        call extend(opts, {'diff': {
+              \ 'oldpath': matchstr(getline(search('^---'.pattern,'bnW')), '^---'.pattern),
+              \ 'newpath': matchstr(getline(search('^+++'.pattern,'bnW')), '^+++'.pattern),
+              \ 'oldline' : offsets['-'] + offsets[' '] + matchstr(getline(lnum), '-\zs\d\+'),
+              \ 'newline' : offsets['+'] + offsets[' '] + matchstr(getline(lnum), '+\zs\d\+'),
+              \ 'cursor': getline('.')[0]}})
+      catch /^Vim\%((\a\+)\)\=:E734/
+      endtry
+    endif
+
     let url = ''
     for Handler in get(g:, 'fugitive_browse_handlers', [])
       let url = call(Handler, [copy(opts)])