revset: optimize missing ancestor expressions
A missing ancestor expression is any expression of the form (::x - ::y) or
equivalent. Such expressions are remarkably common, and so far have involved
multiple walks down the DAG, followed by a set difference operation.
With this patch, such expressions will be transformed into uses of the fast
algorithm at ancestor.missingancestor.
For a repository with over 600,000 revisions, perfrevset for '::tip - ::-10000'
returns:
Before: ! wall 3.999575 comb 4.000000 user 3.910000 sys 0.090000 (best of 3)
After: ! wall 0.132423 comb 0.130000 user 0.130000 sys 0.000000 (best of 75)
Minimal hgk check
$ echo "[extensions]" >> $HGRCPATH
$ echo "hgk=" >> $HGRCPATH
$ hg init repo
$ cd repo
$ echo a > a
$ hg ci -Am adda
adding a
$ hg debug-cat-file commit 0
tree a0c8bcbbb45c
parent 000000000000
author test 0 0
revision 0
branch default
phase draft
adda
$ cd ..