convert: add support for deterministic progress bar on scanning phase

This makes it possible to estimate how long the "scanning source"
phase will take, if the specified source repo type supports a quick
"how many changes" check.

hgext/convert/common.py

@@ -109,6 +109,13 b' class converter_source(object):'
         """Return the commit object for version"""
         raise NotImplementedError
 
+    def numcommits(self):
+        """Return the number of commits in this source.
+
+        If unknown, return None.
+        """
+        return None
+
     def gettags(self):
         """Return the tags as a dictionary of name: revision
 

hgext/convert/convcmd.py

@@ -171,6 +171,7 b' class converter(object):'
         visit = heads
         known = set()
         parents = {}
+        numcommits = self.source.numcommits()
         while visit:
             n = visit.pop(0)
             if n in known:
@@ -180,7 +181,8 b' class converter(object):'
                 if m == SKIPREV or self.dest.hascommitfrommap(m):
                     continue
             known.add(n)
-            self.ui.progress(_('scanning'), len(known), unit=_('revisions'))
+            self.ui.progress(_('scanning'), len(known), unit=_('revisions'),
+                             total=numcommits)
             commit = self.cachecommit(n)
             parents[n] = []
             for p in commit.parents: