merge: introduce 'c' action like 'g', but with additional safety

_checkunknownfile() reads the filelog of the remote side's file. For
narrow clones, the filelog will not exist for all files and we need a
way to avoid reading them. While it would be easier for the narrow
extension to just override _checkunknownfile() and make it ignore
files outside the narrow clone, it seems cleaner to have
manifestmerge() not care about filelogs (considering its
name).

In order to move the calls to _checkunknownfile() out, we need to be
able to tell in which cases we should check for unknown files. Let's
start by introducing a new action distinct from 'g' for this
purpose. Specifically, the new action will be just like 'g' except
that it will check that for conflicting unknown files first. For now,
just add the new action type and convert it to 'g'.

mercurial/merge.py

@@ -488,10 +488,10 b' def manifestmerge(repo, wctx, p2, pa, br'
                 # following table:
                 #
                 # force  branchmerge  different  |  action
-                #   n         *           n      |    get
+                #   n         *           n      |   create
                 #   n         *           y      |   abort
-                #   y         n           *      |    get
-                #   y         y           n      |    get
+                #   y         n           *      |   create
+                #   y         y           n      |   create
                 #   y         y           y      |   merge
                 #
                 # Checking whether the files are different is expensive, so we
@@ -501,9 +501,9 b' def manifestmerge(repo, wctx, p2, pa, br'
                     if different:
                         aborts.append((f, "ud"))
                     else:
-                        actions[f] = ('g', (fl2,), "remote created")
+                        actions[f] = ('c', (fl2,), "remote created")
                 elif not branchmerge:
-                    actions[f] = ('g', (fl2,), "remote created")
+                    actions[f] = ('c', (fl2,), "remote created")
                 else:
                     different = _checkunknownfile(repo, wctx, p2, f)
                     if different:
@@ -517,7 +517,7 b' def manifestmerge(repo, wctx, p2, pa, br'
                     aborts.append((f, 'ud'))
                 else:
                     if acceptremote:
-                        actions[f] = ('g', (fl2,), "remote recreating")
+                        actions[f] = ('c', (fl2,), "remote recreating")
                     else:
                         actions[f] = ('dc', (fl2,), "prompt deleted/changed")
 
@@ -529,6 +529,10 b' def manifestmerge(repo, wctx, p2, pa, br'
         raise util.Abort(_("untracked files in working directory differ "
                            "from files in requested revision"))
 
+    for f, (m, args, msg) in actions.iteritems():
+        if m == 'c':
+            actions[f] = ('g', args, msg)
+
     return actions, diverge, renamedelete
 
 def _resolvetrivial(repo, wctx, mctx, ancestor, actions):