largefiles: introduce "_lfcommithooks" to abstract pre-committing procedures

This changes allows to customize pre-committing procedures according
to conditions.

This patch uses "stack" with an element instead of flag like
"_isXXXXing" or so, because:

  - the former works correctly even when customizations are nested, and
  - ensuring at least one element can ignore empty check

hgext/largefiles/reposetup.py

@@ -262,7 +262,8 b' def reposetup(ui, repo):'
 
             wlock = self.wlock()
             try:
-                match = lfutil.updatestandinsbymatch(self, match)
+                lfcommithook = self._lfcommithooks[-1]
+                match = lfcommithook(self, match)
                 result = orig(text=text, user=user, date=date, match=match,
                                 force=force, editor=editor, extra=extra)
                 return result
@@ -338,6 +339,10 b' def reposetup(ui, repo):'
 
     repo.__class__ = lfilesrepo
 
+    # stack of hooks being executed before committing.
+    # only last element ("_lfcommithooks[-1]") is used for each committing.
+    repo._lfcommithooks = [lfutil.updatestandinsbymatch]
+
     def prepushoutgoinghook(local, remote, outgoing):
         if outgoing.missing:
             toupload = set()