transactions: fix hg recover with fncache backups The transaction backupfiles logic was broken for 'hg recover'. The file format is XXX\0XXX\0YYY\0YYY\0 but the parser did a couple things wrong. 1) It went one step beyond the final \0 and tried to read past the end of the array. 2) array[i:i+1] returns a single item, instead of two items as intended. Added a test to catch it, which turns out to be the first actual 'hg recover' test.