util: move 'readexactly' in the util module

This function is used in multiple place, having it in util would be better.
(existing caller will be migrated in another series)

mercurial/changegroup.py

@@ -32,14 +32,7 b' from . import ('
 _CHANGEGROUPV2_DELTA_HEADER = "20s20s20s20s20s"
 _CHANGEGROUPV3_DELTA_HEADER = ">20s20s20s20s20sH"
 
-def readexactly(stream, n):
-    '''read n bytes from stream.read and abort if less was available'''
-    s = stream.read(n)
-    if len(s) < n:
-        raise error.Abort(_("stream ended unexpectedly"
-                           " (got %d bytes, expected %d)")
-                          % (len(s), n))
-    return s
+readexactly = util.readexactly
 
 def getchunk(stream):
     """return the next chunk from stream as a string"""

mercurial/util.py

@@ -3865,3 +3865,12 b' def safename(f, tag, ctx, others=None):'
         fn = '%s~%s~%s' % (f, tag, n)
         if fn not in ctx and fn not in others:
             return fn
+
+def readexactly(stream, n):
+    '''read n bytes from stream.read and abort if less was available'''
+    s = stream.read(n)
+    if len(s) < n:
+        raise error.Abort(_("stream ended unexpectedly"
+                           " (got %d bytes, expected %d)")
+                          % (len(s), n))
+    return s