merge: use original file extension for temporary files

Some merge tools (like Araxis?) can pick merge mode based on the file
extension. That didn't work well when temporary files were given random
suffixes. It seems to work better when the random part is before the extension.

As usual, when using $output, $local will have the .orig extension. That could
perhaps be the subject of another change another day.

mercurial/filemerge.py

@@ -577,8 +577,9 b' def _filemerge(premerge, repo, mynode, o'
     a boolean indicating whether the file was deleted from disk."""
 
     def temp(prefix, ctx):
-        pre = "%s~%s." % (os.path.basename(ctx.path()), prefix)
-        (fd, name) = tempfile.mkstemp(prefix=pre)
+        fullbase, ext = os.path.splitext(ctx.path())
+        pre = "%s~%s." % (os.path.basename(fullbase), prefix)
+        (fd, name) = tempfile.mkstemp(prefix=pre, suffix=ext)
         data = repo.wwritedata(ctx.path(), ctx.data())
         f = os.fdopen(fd, "wb")
         f.write(data)

tests/test-merge-tools.t

@@ -1209,3 +1209,15 b' internal merge cannot handle symlinks an'
   [1]
 
 #endif
+
+Verify naming of temporary files and that extension is preserved:
+
+  $ hg update -q -C 1
+  $ hg mv f f.txt
+  $ hg ci -qm "f.txt"
+  $ hg update -q -C 2
+  $ hg merge -y -r tip --tool echo --config merge-tools.echo.args='$base $local $other $output'
+  merging f and f.txt to f.txt
+  */f~base.?????? $TESTTMP/f.txt.orig */f~other.??????.txt $TESTTMP/f.txt (glob)
+  0 files updated, 1 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)