largefiles: introduce readasstandin() to read hex hash from given filectx

This will be used to centralize and encapsulate the logic to read hash
from given (filectx of) standin file. readstandin() isn't suitable for
this purpose, because there are some code paths, which want to read
hex hash directly from filectx.

hgext/largefiles/lfutil.py

@@ -355,10 +355,16 b' def updatestandin(repo, lfile, standin):'
     else:
         raise error.Abort(_('%s: file not found!') % lfile)
 
+def readasstandin(fctx):
+    '''read hex hash from given filectx of standin file
+
+    This encapsulates how "standin" data is stored into storage layer.'''
+    return fctx.data().strip()
+
 def readstandin(repo, filename, node=None):
     '''read hex hash from standin for filename at given node, or working
     directory if no node is given'''
-    return repo[node][standin(filename)].data().strip()
+    return readasstandin(repo[node][standin(filename)])
 
 def writestandin(repo, standin, hash, executable):
     '''write hash to <repo.root>/<standin>'''