largefiles: introduce "_lfstatuswriters" to customize status reporting

"lfutil.getstatuswriter" is the utility to get appropriate function to
write largefiles specific status out from "repo._lfstatuswriters".

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/lfutil.py

@@ -557,3 +557,20 b' class automatedcommithook(object):'
             return updatestandinsbymatch(repo, match)
         else:
             return match
+
+def getstatuswriter(ui, repo, forcibly=None):
+    '''Return the function to write largefiles specific status out
+
+    If ``forcibly`` is ``None``, this returns the last element of
+    ``repo._lfupdatereporters`` as "default" writer function.
+
+    Otherwise, this returns the function to always write out (or
+    ignore if ``not forcibly``) status.
+    '''
+    if forcibly is None:
+        return repo._lfstatuswriters[-1]
+    else:
+        if forcibly:
+            return ui.status # forcibly WRITE OUT
+        else:
+            return lambda *msg, **opts: None # forcibly IGNORE

hgext/largefiles/reposetup.py

@@ -343,6 +343,11 b' def reposetup(ui, repo):'
     # only last element ("_lfcommithooks[-1]") is used for each committing.
     repo._lfcommithooks = [lfutil.updatestandinsbymatch]
 
+    # Stack of status writer functions taking "*msg, **opts" arguments
+    # like "ui.status()". Only last element ("_lfupdatereporters[-1]")
+    # is used to write status out.
+    repo._lfstatuswriters = [ui.status]
+
     def prepushoutgoinghook(local, remote, outgoing):
         if outgoing.missing:
             toupload = set()