histedit: use base for computing renames when folding (issue3729)

When a file was renamed in the very first changeset being histedited,
the rename was not detected. Consider a history like this:

   0  base: add a.txt
   1  first: rename a.txt to b.txt
   2  last: edit b.txt

When 1 and 2 are edited, copies.pathcopies(first, last) correctly
returns that nothing was renamed. We must instead use
copies.pathcopies(first, last) to detect the initial rename.

hgext/histedit.py

@@ -247,7 +247,7 b' def collapse(repo, first, last, commitop'
         files.update(ctx.files())
 
     # Recompute copies (avoid recording a -> b -> a)
-    copied = copies.pathcopies(first, last)
+    copied = copies.pathcopies(base, last)
 
     # prune files which were reverted by the updates
     def samefile(f):

tests/test-histedit-fold.t

@@ -266,3 +266,37 b' dropped revision.'
   +6
   $ cd ..
 
+
+Folding with initial rename (issue3729)
+---------------------------------------
+
+  $ hg init fold-rename
+  $ cd fold-rename
+  $ echo a > a.txt
+  $ hg add a.txt
+  $ hg commit -m a
+  $ hg rename a.txt b.txt
+  $ hg commit -m rename
+  $ echo b >> b.txt
+  $ hg commit -m b
+
+  $ hg logt --follow b.txt
+  2:e0371e0426bc b
+  1:1c4f440a8085 rename
+  0:6c795aa153cb a
+
+  $ hg histedit 1c4f440a8085 --commands - 2>&1 << EOF | fixbundle
+  > pick 1c4f440a8085 rename
+  > fold e0371e0426bc b
+  > EOF
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  reverting b.txt
+  1 files updated, 0 files merged, 1 files removed, 0 files unresolved
+  1 files updated, 0 files merged, 1 files removed, 0 files unresolved
+  0 files updated, 0 files merged, 0 files removed, 0 files unresolved
+
+  $ hg logt --follow b.txt
+  1:cf858d235c76 rename
+  0:6c795aa153cb a
+
+  $ cd ..