fix parsing of tags. make parse errors useful. add new tag tests.
old code read every head of .hgtags. delete and recreate of .hgtags gave
new head, but if error in deleted rev, .hgtags had error messages every
time it was parsed. this was very hard to fix, because deleted revs hard
to get back and update, needed merges too.
new code reads .hgtags on every head. advantage is if parse error
happens with new code, is possible to fix them by editing .hgtags on a
head and committing.
NOTE: new code uses binary search of manifest of each head to be fast,
but still much slower than old code. best thing would be to have delete
record stored in filelog so we never touch manifest. could find live
heads directly from filelog. this is more work than i want now.
new tests check for parse of tags on different heads, and inaccessible
heads created by delete and recreate of .hgtags.