resolve: don't abort when file is missing

A file being missing is a completely valid situation in which the user may want
to re-resolve merge conflicts. Mercurial already maintains backups of local
data, so this turns out to be easy to handle.

mercurial/commands.py

@@ -5655,7 +5655,11 b' def resolve(ui, repo, *pats, **opts):'
             else:
                 # backup pre-resolve (merge uses .orig for its own purposes)
                 a = repo.wjoin(f)
-                util.copyfile(a, a + ".resolve")
+                try:
+                    util.copyfile(a, a + ".resolve")
+                except (IOError, OSError) as inst:
+                    if inst.errno != errno.ENOENT:
+                        raise
 
                 try:
                     # preresolve file
@@ -5673,7 +5677,11 b' def resolve(ui, repo, *pats, **opts):'
                 # replace filemerge's .orig file with our resolve file
                 # for files in tocomplete, ms.resolve will not overwrite
                 # .orig -- only preresolve does
-                util.rename(a + ".resolve", a + ".orig")
+                try:
+                    util.rename(a + ".resolve", a + ".orig")
+                except OSError as inst:
+                    if inst.errno != errno.ENOENT:
+                        raise
 
         for f in tocomplete:
             try:

tests/test-merge-tools.t

@@ -579,6 +579,33 b' prompt with EOF'
   ? f.orig
   # hg resolve --list
   U f
+  $ rm f
+  $ hg resolve --all --config ui.merge=internal:prompt --config ui.interactive=true
+   no tool found to merge f
+  keep (l)ocal or take (o)ther? 
+  [1]
+  $ aftermerge
+  # cat f
+  revision 1
+  space
+  # hg stat
+  M f
+  # hg resolve --list
+  U f
+  $ hg resolve --all --config ui.merge=internal:prompt
+   no tool found to merge f
+  keep (l)ocal or take (o)ther? l
+  (no more unresolved files)
+  $ aftermerge
+  # cat f
+  revision 1
+  space
+  # hg stat
+  M f
+  ? f.orig
+  # hg resolve --list
+  R f
+
 ui.merge specifies internal:dump:
 
   $ beforemerge