commitctx: reorder some conditional for efficiency in _filecommit

Checking if a dict is empty will be faster than comparing text. I don't expect
it to be a huge performance win, but still a good (but gratuitous) cleanup to
do while we are at it.

This is part of a larger refactoring/cleanup of the commitctx code to clarify
and augment the logic gathering metadata useful for copy tracing. The current
code is a tad too long and entangled to make such update easy. We start with
easy and small cleanup.

Differential Revision: https://phab.mercurial-scm.org/D8701

mercurial/localrepo.py

@@ -2899,7 +2899,7 b' class localrepository(object):'
 
         # is the file changed?
         text = fctx.data()
-        if fparent2 != nullid or flog.cmp(fparent1, text) or meta:
+        if fparent2 != nullid or meta or flog.cmp(fparent1, text):
             changelist.append(fname)
             return flog.add(text, meta, tr, linkrev, fparent1, fparent2)
         # are just the flags changed during merge?