merge: warn when internal:merge cannot merge symlinks

A follow-up to d084df89d948.

internal:merge should never be picked for merging symlinks ... but in the test
suite we have HGMERGE="internal:merge" which bypasses all the usual merge-tool
cleverness. Without any output it can be hard to figure out what happened and
where the problem is.

mercurial/filemerge.py

@@ -208,6 +208,8 b' def _imerge(repo, mynode, orig, fcd, fco'
     the partially merged file."""
     tool, toolpath, binary, symlink = toolconf
     if symlink:
+        repo.ui.warn(_('warning: internal:merge cannot merge symlinks '
+                       'for %s\n') % fcd.path())
         return False, 1
 
     r = _premerge(repo, toolconf, files)

tests/test-merge-tools.t

@@ -843,6 +843,7 b' internal merge cannot handle symlinks an'
   $ hg commit -qm 'f is symlink'
   $ hg merge -r 2 --tool internal:merge
   merging f
+  warning: internal:merge cannot merge symlinks for f
   merging f incomplete! (edit conflicts, then use 'hg resolve --mark')
   0 files updated, 0 files merged, 0 files removed, 1 files unresolved
   use 'hg resolve' to retry unresolved file merges or 'hg update -C .' to abandon