convert: fix bad conversion of copies when hg.startrev is specified

The 'copynode' was looked up in self.keep as if it was a changeset node. It is
however a filelog node, and self.keep would thus fail if it actually looked at
its parameter ... which it only did if a startrev was specified.

Instead we now don't check the copy node - we don't have to. It must have been
copied from one of the parents, and we already check whether one of the parents
have the copy source.

We could perhaps use linkrev to see if the corresponding changeset was
converted ... but that would sometimes be wrong.

The existing test of this was wrong - now it is better, but it seems like it
exposes a 'log' issue.

hgext/convert/hg.py

@@ -316,8 +316,8 b' class mercurial_source(converter_source)'
             if name in self.ignored:
                 continue
             try:
-                copysource, copynode = ctx.filectx(name).renamed()
-                if copysource in self.ignored or not self.keep(copynode):
+                copysource, _copynode = ctx.filectx(name).renamed()
+                if copysource in self.ignored:
                     continue
                 # Ignore copy sources not in parent revisions
                 found = False

tests/test-convert-hg-startrev.t

@@ -120,16 +120,26 b' Convert from merge parent'
 
 Check copy preservation
 
+  $ hg st -C --change 2 e
+  M e
+  $ hg st -C --change 1 e
+  A e
+    a
+  $ hg st -C --change 0 a
+  A a
+
+(It seems like a bug in log that the following doesn't show rev 1.)
+
   $ hg log --follow --copies e
-  changeset:   2:60633ee11cfa
+  changeset:   2:82bbac3d2cf4
   user:        test
   date:        Thu Jan 01 00:00:04 1970 +0000
   summary:     4: merge 2 and 3
   
-  changeset:   1:d56e8baefff8
+  changeset:   0:23c3be426dce
   user:        test
-  date:        Thu Jan 01 00:00:02 1970 +0000
-  summary:     2: copy e from a, change b
+  date:        Thu Jan 01 00:00:01 1970 +0000
+  summary:     1: add c, move f to d
   
 Check copy removal on missing parent