tests/test-rebuildstate.out

store all heads of a branch in the branch cache

All heads of branches will be stored in a new cache file 'branchheads.cache'
within the .hg directory. The old 'branch.cache' file from older versions
will be ignored.

The new cache contents are formatted line-by-line as '{node} {branchtag}\n'.
This is the same as the previous format. Now, every head is recorded in
an oldest -> tipmost order.

The localrepo.branchheads function is reworked to use the data from the cache.

adding bar
adding foo
% state dump after
n 666         -1 bar
n 666         -1 foo
% status
! bar
? baz
C foo