patch: don't look for headers in diff lines
If you have a diff line that matches a header line, the patch splitter
currently breaks your patch at this line. For example a line like:
+key: value
This can lead to "malformed patch" exceptions. Now fixed.
localrepo: add desc parameter to transaction
All callers to localrepo.transaction() must supply a transaction description.
The description and the existing repository tip are then stored
(transactionally) into .hg/undo.desc; where rollback can later find it.