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.
This should avoid a bad performance problem when the branch cache is
not up-to-date, and hgweb can't write an updated version because it
lacks permissions.
The leading space in the written file makes sure that the feature list never
can match an existing version, even if the first feature can be read as hex.
Additionally old hg versions display the id with --debug, too.
Operations on this branch don't work otherwise.
Reading branches.cache had to be adjusted to allow an empty label.
Adjusted reading of the cache tip for symmetry, no functional change here.
- hg tip doesn't use the branch cache; use hg log -r in its place
- commit doesn't use a HG_MERGE environment variable
- change the dates from "0 0" to "1000000 0"
