filemerge: fix backing up an in-memory file to a custom location

If the user specifies a ui.origbackuppath, we used to always copy the file
there, but if the source file is in memory we must write it instead of copying.

Differential Revision: https://phab.mercurial-scm.org/D1806

mercurial/filemerge.py

@@ -618,6 +618,9 b' def _makebackup(repo, ui, wctx, fcd, pre'
     (if any), the backup is used to undo certain premerges, confirm whether a
     merge changed anything, and determine what line endings the new file should
     have.
+
+    Backups only need to be written once (right before the premerge) since their
+    content doesn't change afterwards.
     """
     if fcd.isabsent():
         return None
@@ -628,7 +631,6 b' def _makebackup(repo, ui, wctx, fcd, pre'
     back = scmutil.origpath(ui, repo, a)
     inworkingdir = (back.startswith(repo.wvfs.base) and not
         back.startswith(repo.vfs.base))
-
     if isinstance(fcd, context.overlayworkingfilectx) and inworkingdir:
         # If the backup file is to be in the working directory, and we're
         # merging in-memory, we must redirect the backup to the memory context
@@ -637,12 +639,17 b' def _makebackup(repo, ui, wctx, fcd, pre'
         wctx[relpath].write(fcd.data(), fcd.flags())
         return wctx[relpath]
     else:
-        # Otherwise, write to wherever the user specified the backups should go.
-        #
+        if premerge:
+            # Otherwise, write to wherever path the user specified the backups
+            # should go. We still need to switch based on whether the source is
+            # in-memory so we can use the fast path of ``util.copy`` if both are
+            # on disk.
+            if isinstance(fcd, context.overlayworkingfilectx):
+                util.writefile(back, fcd.data())
+            else:
+                util.copyfile(a, back)
         # A arbitraryfilectx is returned, so we can run the same functions on
         # the backup context regardless of where it lives.
-        if premerge:
-            util.copyfile(a, back)
         return context.arbitraryfilectx(back, repo=repo)
 
 def _maketempfiles(repo, fco, fca):