filemerge: store error messages in module variables

Copytracing may be disabled because it's too slow (see
experimental.disablecopytrace config option). In that case user may get errors
like 'local changed FILE which other deleted'. It would be nice to give user a
hint to rerun command with `--config experimental.disablecopytrace=False`. To
make it possible let's extract error message to variables so that extension may
overwrite them.

mercurial/filemerge.py

@@ -49,6 +49,17 b' nomerge = None'
 mergeonly = 'mergeonly'  # just the full merge, no premerge
 fullmerge = 'fullmerge'  # both premerge and merge
 
+_localchangedotherdeletedmsg = _(
+    "local%(l)s changed %(fd)s which other%(o)s deleted\n"
+    "use (c)hanged version, (d)elete, or leave (u)nresolved?"
+    "$$ &Changed $$ &Delete $$ &Unresolved")
+
+_otherchangedlocaldeletedmsg = _(
+    "other%(o)s changed %(fd)s which local%(l)s deleted\n"
+    "use (c)hanged version, leave (d)eleted, or "
+    "leave (u)nresolved?"
+    "$$ &Changed $$ &Deleted $$ &Unresolved")
+
 class absentfilectx(object):
     """Represents a file that's ostensibly in a context but is actually not
     present in it.
@@ -250,16 +261,11 b' def _iprompt(repo, mynode, orig, fcd, fc'
     try:
         if fco.isabsent():
             index = ui.promptchoice(
-                _("local%(l)s changed %(fd)s which other%(o)s deleted\n"
-                  "use (c)hanged version, (d)elete, or leave (u)nresolved?"
-                  "$$ &Changed $$ &Delete $$ &Unresolved") % prompts, 2)
+                _localchangedotherdeletedmsg % prompts, 2)
             choice = ['local', 'other', 'unresolved'][index]
         elif fcd.isabsent():
             index = ui.promptchoice(
-                _("other%(o)s changed %(fd)s which local%(l)s deleted\n"
-                  "use (c)hanged version, leave (d)eleted, or "
-                  "leave (u)nresolved?"
-                  "$$ &Changed $$ &Deleted $$ &Unresolved") % prompts, 2)
+                _otherchangedlocaldeletedmsg % prompts, 2)
             choice = ['other', 'local', 'unresolved'][index]
         else:
             index = ui.promptchoice(