convert: allow passing in a revmap

Implement `common.setrevmap` which is used to pass in a file with existing
revision mappings. This functionality is used by `convertcmd.convert` if it
exists and allows implementors such as the p4 converter to make use of an
existing mapping.

We are using the revmap to abort scanning and the repository for more information
if we already have the revision. This means we are allowing incremental imports
in cases where a revmap is provided.

hgext/convert/p4.py

@@ -55,6 +55,7 b' class p4_source(common.converter_source)'
 
         common.checktool('p4', abort=False)
 
+        self.revmap = {}
         self.p4changes = {}
         self.heads = []
         self.changeset = {}
@@ -77,6 +78,19 b' class p4_source(common.converter_source)'
                                "multiple revisions"))
         self._parse(ui, path)
 
+    def setrevmap(self, revmap):
+        """Sets the parsed revmap dictionary.
+
+        Revmap stores mappings from a source revision to a target revision.
+        It is set in convertcmd.convert and provided by the user as a file
+        on the commandline.
+
+        Revisions in the map are considered beeing present in the
+        repository and ignored during _parse(). This allows for incremental
+        imports if a revmap is provided.
+        """
+        self.revmap = revmap
+
     def _parse_view(self, path):
         "Read changes affecting the path"
         cmd = 'p4 -G changes -s submitted %s' % util.shellquote(path)
@@ -132,6 +146,10 b' class p4_source(common.converter_source)'
                 continue
             if self.revs and int(change) > int(self.revs[0]):
                 continue
+            if change in self.revmap:
+                # Ignore already present revisions, but set the parent pointer.
+                lastid = change
+                continue
 
             cmd = "p4 -G describe -s %s" % change
             stdout = util.popen(cmd, mode='rb')