revset: remove no longer used _missingancestors revset This was undocumented.

revset: replace _missingancestors optimization with only revset (::a - ::b) is equivalent to only(a, b).

tags: introduce _readtaghist function The existing _readtags function has been modified a little and renamed _readtaghist. A new _readtaghist function has been added, which is a wrappger around _readtaghist. Its output is the same as the old _readtaghist. The purpose of this change is to make it possible to automatically merge tag files. In order to do so we will need to get the line numbers for each of the tag-node pairs on the first merge parent. This is not used yet, but will be used on a follow up patch that will introduce an automatic tag merge algorithm. I performed some tests to compare the effect of this change. I used timeit to run the test-tags.t test a 9 times with and without this patch. The results were: - without this patch: 3 loops, best of 3: 8.55 sec per loop - with this patch: 3 loops, best of 3: 8.49 sec per loop The the test was on average was slightly faster with this patch (although the difference was probably not statistically significant).