branchmap: avoid ancestor computations in absence of non-continous branches The branchhead computation is one of the more heavy operations for bigger repositories as it has to scan all changesets and potentially involves the expensive computation of the ancestor sets. Redo the computation to handle the common cases directly and use tighter conditions for when the ancestor scan is necessary. Most importantly, avoid it completely if the non-continous branches are processed in one update as seen in the initial computation after a clone. For the Mercurial repository, it gives a small 2-3% performance boost. For the NetBSD test repository, it cuts the time in half. Differential Revision: https://phab.mercurial-scm.org/D9631

--- a/relnotes/next	Tue Jan 12 19:49:18 2021 +0100
+++ b/relnotes/next	Fri Dec 18 14:45:28 2020 +0100
@@ -39,6 +39,10 @@
 
  * External hooks are now called with `HGPLAIN=1` preset.
 
+ * The `branchmap` cache is updated more intelligently and can be
+   significantly faster for repositories with many branches and changesets.
+
+
 == New Experimental Features ==
 
 * `experimental.single-head-per-branch:public-changes-only` can be used