Respect 'splitbelow' on :Gdiffsplit with argument

When calling :Gdiffsplit with no argument, we always end up with the
work tree version as half of the diff, and it is helpful to position
that consistently.  I generalized this to a consistent older versus
newer ordering when given an argument, but I don't think that has proven
very useful in practice.

This also introduces a minor behavior change where calling the bang
variant in the initial commit now loads an empty version of the buffer,
rather than falling back to the work tree.

autoload/fugitive.vim

@@ -6116,28 +6116,6 @@ function! s:diffoff_all(dir) abort
   diffoff!
 endfunction
 
-function! s:CompareAge(mine, theirs) abort
-  let scores = {':0': 1, ':1': 2, ':2': 3, ':': 4, ':3': 5}
-  let mine = substitute(a:mine, '^:', '', '')
-  let theirs = substitute(a:theirs, '^:', '', '')
-  let my_score    = get(scores, ':'.mine, 0)
-  let their_score = get(scores, ':'.theirs, 0)
-  if my_score || their_score
-    return my_score < their_score ? -1 : my_score != their_score
-  elseif mine ==# theirs
-    return 0
-  endif
-  let base = s:TreeChomp('merge-base', mine, theirs)
-  if base ==# mine
-    return -1
-  elseif base ==# theirs
-    return 1
-  endif
-  let my_time    = +s:TreeChomp('log', '--max-count=1', '--pretty=format:%at', a:mine, '--')
-  let their_time = +s:TreeChomp('log', '--max-count=1', '--pretty=format:%at', a:theirs, '--')
-  return my_time < their_time ? -1 : my_time != their_time
-endfunction
-
 function! s:IsConflicted() abort
   return len(@%) && !empty(s:ChompDefault('', ['ls-files', '--unmerged', '--', expand('%:p')]))
 endfunction
@@ -6218,7 +6196,7 @@ function! fugitive#Diffsplit(autodir, keepfocus, mods, arg, args) abort
         let file = s:Relative()
       elseif arg ==# ':'
         exe s:DirCheck()
-        let file = s:Relative(':0:')
+        let file = len(commit) ? s:Relative() : s:Relative(s:IsConflicted() ? ':1:' : ':0:')
       elseif arg =~# '^:\d$'
         exe s:DirCheck()
         let file = s:Relative(arg . ':')
@@ -6231,18 +6209,23 @@ function! fugitive#Diffsplit(autodir, keepfocus, mods, arg, args) abort
           return 'echoerr ' . string(v:exception)
         endtry
       endif
-    elseif exists('parents') && len(parents)
+      let mods = s:Mods(a:mods)
-      let file = parents[-1]
+    elseif exists('parents')
+      let file = get(parents, -1, s:Relative(repeat('0', 40). ':'))
+      let mods = s:Mods(a:mods, 'leftabove')
     elseif len(commit)
       let file = s:Relative()
+      let mods = s:Mods(a:mods, 'rightbelow')
     elseif s:IsConflicted()
       let file = s:Relative(':1:')
+      let mods = s:Mods(a:mods, 'leftabove')
       if get(g:, 'fugitive_legacy_commands', 1)
         let post = 'echohl WarningMsg|echo "Use :Gdiffsplit! for 3 way diff"|echohl NONE|' . post
       endif
     else
       exe s:DirCheck()
       let file = s:Relative(':0:')
+      let mods = s:Mods(a:mods, 'leftabove')
     endif
     let spec = s:Generate(file)
     if spec =~# '^fugitive:' && empty(s:DirCommitFile(spec)[2])
@@ -6251,11 +6234,6 @@ function! fugitive#Diffsplit(autodir, keepfocus, mods, arg, args) abort
     exe pre
     let restore = s:diff_restore()
     let w:fugitive_diff_restore = restore
-    if len(spec) && s:CompareAge(commit, s:DirCommitFile(spec)[1]) < 0
-      let mods = s:Mods(mods, 'rightbelow')
-    else
-      let mods = s:Mods(mods, 'leftabove')
-    endif
     let mods = (autodir ? s:diff_modifier(2) : '') . mods
     if &diffopt =~# 'vertical'
       let diffopt = &diffopt

doc/fugitive.txt

@@ -184,10 +184,11 @@ that are part of Git repositories).
 :Gdiffsplit [object]    Perform a |vimdiff| against the given file, or if a
                         commit is given, the current file in that commit.
                         With no argument, the version in the index or work
-                        tree is used.  The newer of the two files is placed to
+                        tree is used, and the work tree version is always
-                        the right or bottom, depending on 'diffopt' and the
+                        placed to the right or bottom.  A vertical split is
-                        width of the window relative to 'textwidth'.  Use
+                        used if space permits, and if 'diffopt' does not
-                        Vim's |do| and |dp| to stage and unstage changes.
+                        otherwise specify.  Use Vim's |do| and |dp| to stage
+                        and unstage changes.
 
                                                 *:Gdiffsplit!*
 :Gdiffsplit!            Diff against any and all direct ancestors, retaining