changelog: fix handling of empty copy entries in changeset

Before this patch, when an empty value was found in the changeset, we
would get a ValueError, which would result in None being returned for
addedfiles/removedfiles and p1copies/p2copies. That made 278dcb24e535
(copies: write empty entries in changeset when also writing to
filelog, 2019-04-23) ineffective at helping the read path not look for
copies in the filelogs.

Differential Revision: https://phab.mercurial-scm.org/D6595

mercurial/changelog.py

@@ -92,6 +92,8 b' def encodecopies(files, copies):'
 def decodecopies(files, data):
     try:
         copies = {}
+        if not data:
+            return copies
         for l in data.split('\n'):
             strindex, src = l.split('\0')
             i = int(strindex)
@@ -114,6 +116,8 b' def encodefileindices(files, subset):'
 def decodefileindices(files, data):
     try:
         subset = []
+        if not data:
+            return subset
         for strindex in data.split('\n'):
             i = int(strindex)
             if i < 0 or i >= len(files):