merge: report all files in _checkunknown

When doing hg up, if there is a file conflict with untracked files,
currently only the first such conflict is reported. With this patch,
all of them are listed.

With this patch error message is now reported as

    a: untracked file differs
    b: untracked file differs
    abort: untracked files in working directory conflict with files in
    requested revision

instead of

    abort: untracked file in working directory differs from file in
    requested revision: 'a'

This is a follow up to an old attempt to do this here:

    http://selenic.com/pipermail/mercurial-devel/2011-August/033625.html

mercurial/merge.py

@@ -90,11 +90,16 b' def _checkunknown(wctx, mctx, folding):'
     folded = {}
     for fn in mctx:
         folded[foldf(fn)] = fn
+
+    error = False
     for fn in wctx.unknown():
         f = foldf(fn)
         if f in folded and mctx[folded[f]].cmp(wctx[f]):
-            raise util.Abort(_("untracked file in working directory differs"
-                               " from file in requested revision: '%s'") % fn)
+            error = True
+            wctx._repo.ui.warn(_("%s: untracked file differs\n") % fn)
+    if error:
+        raise util.Abort(_("untracked files in working directory differ "
+                           "from files in requested revision"))
 
 def _checkcollision(mctx, wctx):
     "check for case folding collisions in the destination context"

tests/test-casefolding.t

@@ -66,7 +66,8 b' no clobbering of untracked files with wr'
   0 files updated, 0 files merged, 1 files removed, 0 files unresolved
   $ echo gold > a
   $ hg up
-  abort: untracked file in working directory differs from file in requested revision: 'a'
+  a: untracked file differs
+  abort: untracked files in working directory differ from files in requested revision
   [255]
   $ cat a
   gold

tests/test-merge1.t

@@ -66,7 +66,8 b' no merges expected'
   $ echo This is file b2 > b
 merge should fail
   $ hg merge 1
-  abort: untracked file in working directory differs from file in requested revision: 'b'
+  b: untracked file differs
+  abort: untracked files in working directory differ from files in requested revision
   [255]
 merge of b expected
   $ hg merge -f 1