filemerge: use arbitraryfilectx for backups

 With in-memory merge, backup files might be overlayworkingfilectxs stored
in memory. But they could also be real files if the user's backup directory is
outside the working dir.

Rather than have two code paths everywhere, let's use arbitraryfilectx so they
can be consistent.

Differential Revision: https://phab.mercurial-scm.org/D1057

mercurial/filemerge.py

@@ -7,7 +7,6 b''
 
 from __future__ import absolute_import
 
-import filecmp
 import os
 import re
 import tempfile
@@ -226,9 +225,9 b' def _eoltype(data):'
         return '\n'
     return None # unknown
 
-def _matcheol(file, origfile):
+def _matcheol(file, back):
     "Convert EOL markers in a file to match origfile"
-    tostyle = _eoltype(util.readfile(origfile))
+    tostyle = _eoltype(back.data()) # No repo.wread filters?
     if tostyle:
         data = util.readfile(file)
         style = _eoltype(data)
@@ -505,7 +504,9 b' def _xmerge(repo, mynode, orig, fcd, fco'
 
         args = _toolstr(ui, tool, "args", '$local $base $other')
         if "$output" in args:
-            out, a = a, back # read input from backup, write to original
+            # read input from backup, write to original
+            out = a
+            a = repo.wvfs.join(back.path())
         replace = {'local': a, 'base': b, 'other': c, 'output': out}
         args = util.interpolate(r'\$', replace, args,
                                 lambda s: util.shellquote(util.localpath(s)))
@@ -588,10 +589,10 b' def partextras(labels):'
 def _restorebackup(fcd, back):
     # TODO: Add a workingfilectx.write(otherfilectx) path so we can use
     # util.copy here instead.
-    fcd.write(util.readfile(back), fcd.flags())
+    fcd.write(back.data(), fcd.flags())
 
-def _makebackup(repo, ui, fcd, premerge):
-    """Makes a backup of the local `fcd` file prior to merging.
+def _makebackup(repo, ui, wctx, fcd, premerge):
+    """Makes and returns a filectx-like object for ``fcd``'s backup file.
 
     In addition to preserving the user's pre-existing modifications to `fcd`
     (if any), the backup is used to undo certain premerges, confirm whether a
@@ -600,12 +601,14 b' def _makebackup(repo, ui, fcd, premerge)'
     """
     if fcd.isabsent():
         return None
-
+    # TODO: Break this import cycle somehow. (filectx -> ctx -> fileset ->
+    # merge -> filemerge). (I suspect the fileset import is the weakest link)
+    from . import context
     a = _workingpath(repo, fcd)
     back = scmutil.origpath(ui, repo, a)
     if premerge:
         util.copyfile(a, back)
-    return back
+    return context.arbitraryfilectx(back, repo=repo)
 
 def _maketempfiles(repo, fco, fca):
     """Writes out `fco` and `fca` as temporary files, so an external merge
@@ -691,7 +694,7 b' def _filemerge(premerge, repo, wctx, myn'
             ui.warn(onfailure % fd)
         return True, 1, False
 
-    back = _makebackup(repo, ui, fcd, premerge)
+    back = _makebackup(repo, ui, wctx, fcd, premerge)
     files = (None, None, None, back)
     r = 1
     try:
@@ -719,7 +722,7 b' def _filemerge(premerge, repo, wctx, myn'
         return True, r, deleted
     finally:
         if not r and back is not None:
-            util.unlink(back)
+            back.remove()
 
 def _check(repo, r, ui, tool, fcd, files):
     fd = fcd.path()
@@ -741,7 +744,7 b' def _check(repo, r, ui, tool, fcd, files'
     if not r and not checked and (_toolbool(ui, tool, "checkchanged") or
                                   'changed' in
                                   _toollist(ui, tool, "check")):
-        if back is not None and filecmp.cmp(_workingpath(repo, fcd), back):
+        if back is not None and not fcd.cmp(back):
             if ui.promptchoice(_(" output file %s appears unchanged\n"
                                  "was merge successful (yn)?"
                                  "$$ &Yes $$ &No") % fd, 1):