Test that we notice the empty files from the second parent during a merge

This didn't work with 0.9.1, but was fixed by 05257fd28591.

tests/test-empty-file

@@ -0,0 +1,25 b''
+#!/bin/sh
+
+hg init a
+cd a
+touch empty1
+hg add empty1
+hg commit -m 'add empty1' -d '1000000 0'
+
+touch empty2
+hg add empty2
+hg commit -m 'add empty2' -d '1000000 0'
+
+hg up -C 0
+touch empty3
+hg add empty3
+hg commit -m 'add empty3' -d '1000000 0'
+
+hg heads
+
+hg merge 1
+# before changeset 05257fd28591, we didn't notice the 
+# empty file that came from rev 1.
+hg status
+hg commit -m merge -d '1000000 0'
+hg manifest tip

tests/test-empty-file.out

@@ -0,0 +1,19 b''
+0 files updated, 0 files merged, 1 files removed, 0 files unresolved
+changeset:   2:62ec0e86d1e5
+tag:         tip
+parent:      0:567dde5e6e98
+user:        test
+date:        Mon Jan 12 13:46:40 1970 +0000
+summary:     add empty3
+
+changeset:   1:41ab7b321727
+user:        test
+date:        Mon Jan 12 13:46:40 1970 +0000
+summary:     add empty2
+
+1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+(branch merge, don't forget to commit)
+M empty2
+b80de5d138758541c5f05265ad144ab9fa86d1db 644 empty1
+b80de5d138758541c5f05265ad144ab9fa86d1db 644 empty2
+b80de5d138758541c5f05265ad144ab9fa86d1db 644 empty3