Support any scm by assigning args at startup

Fix #13: add g:mergetool_args_order.

Add g:mergetool_args_order to renames arguments as hidden files with the
same names as git (BASE, REMOTE, LOCAL). This allows mergetool to behave
as if invoked by git-mergetool for any scm.

autoload/mergetool.vim

@@ -6,6 +6,7 @@ endfunction
 let g:mergetool_layout = get(g:, 'mergetool_layout', 'mr')
 let g:mergetool_prefer_revision = get(g:, 'mergetool_prefer_revision', 'local')
 let g:MergetoolSetLayoutCallback = get(g:, 'MergetoolSetLayoutCallback', function('s:noop'))
+let g:mergetool_args_order = get(g:, 'mergetool_args_order', '')
 
 " {{{ Public exports
 
@@ -38,6 +39,16 @@ function! mergetool#start() "{{{
   let s:mergedfile_fileformat = &fileformat
   let s:mergedfile_filetype = &filetype
 
+  if !empty(g:mergetool_args_order)
+    let success = s:apply_args_order(s:mergedfile_bufnr, g:mergetool_args_order)
+    if !success
+      echohl WarningMsg
+      echo "g:mergetool_args_order didn't use the current file as MERGED. Ensure you're using the order as seen in :args."
+      echohl None
+      return
+    endif
+  endif
+
   " Detect if we're run as 'git mergetool' by presence of BASE|LOCAL|REMOTE buf names
   let s:run_as_git_mergetool = bufnr('BASE') != -1 &&
         \ bufnr('LOCAL') != -1 &&
@@ -117,6 +128,38 @@ function! mergetool#toggle() " {{{
   endif
 endfunction " }}}
 
+" Create hidden buffers that use git's special buffer names to support any
+" scm. We never create a MERGED buffer. Instead, return it so we can validate
+" it's as expected.
+function! s:apply_args_order(merged_bufnr, arg_order) " {{{
+  let abbrevs = {
+        \ 'M': 'MERGED',
+        \ 'B': 'BASE',
+        \ 'R': 'REMOTE',
+        \ 'L': 'LOCAL' }
+
+  let i = 1
+  for labbr in split(a:arg_order, '\zs')
+    if labbr ==# 'M'
+      let current_arg_bufnr = bufnr(argv(i - 1))
+      if a:merged_bufnr != current_arg_bufnr
+        " Fail -- input merged buffer number doesn't match arg order.
+        return 0
+      endif
+    else
+      execute 'silent' i 'argument'
+      execute 'silent file' abbrevs[labbr]
+      setlocal buftype=nofile
+      setlocal bufhidden=hide
+    endif
+    let i += 1
+  endfor
+
+  execute "buffer " . a:merged_bufnr
+  " Success
+  return 1
+endfunction " }}}
+
 " Opens set of windows with merged file and various file revisions
 " Supported layout options:
 " - w, 'MERGED' revision as passed by Git, or working tree version of merged file

readme.md

@@ -275,6 +275,41 @@ Git detects whether merge was successful or not in two ways:
 `vim-mergetool` supports both options. On quit, if merge was unsuccessful,  it both discards any unsaved changes to buffer without touching file's `ctime` and returns non-zero exit code.
 
 
+### Running as other scm mergetool
+
+You can set the g:mergetool_args_order variable when you start vim to tell vim-mergetool that your arguments are the files to use for merging. Setup your scm to start vim like this:
+
+    gvim --nofork -c "let g:mergetool_args_order = 'MBRL'" -c "MergetoolStart" $MERGED $BASE $REMOTE $LOCAL
+
+**MERGED should be the first file** because MergetoolStart is only valid in a file with conflict markers.
+
+Your scm likely has its own names for these filenames. Check your documentation.
+
+
+#### Example: Subversion
+
+Subversion names the files something like this:
+
+* MERGED --> file.vim
+* BASE --> file.vim.r404217
+* REMOTE --> file.vim.r404563
+* LOCAL --> file.vim.mine
+
+So you'd start a merge like this:
+
+    gvim --nofork -c "let g:mergetool_args_order = 'MBRL'" -c "MergetoolStart" file.vim file.vim.r404217 file.vim.r404563 file.vim.mine
+
+vim-mergetool will act like it does as a git-mergetool (no extra tab and won't try to access git to load other files).
+
+For TortoiseSVN, create a batchfile like this and set it as your mergetool:
+
+    set LOCAL=%1
+    set REMOTE=%2
+    set BASE=%3
+    set MERGED=%4
+    gvim --nofork -c "let g:mergetool_args_order = 'MBLR'" -c "Merge" "%MERGED%" "%BASE%" "%LOCAL%" "%REMOTE%"
+
+
 ### Running directly from running Vim instance
 You can enter and exit merge mode from running Vim instance by opening a file with conflict markers, and running one of the commands: