context: generate filtered manifest efficiently for exact matchers
When the matcher is exact, there's no reason to iterate over the entire
manifest. It's much more efficient to iterate over the list of files instead.
For a repository with approximately 300,000 files, this speeds up
hg log -l10 --patch --follow for a frequently modified file from 16.5 seconds
to 10.5 seconds.
$ rm -rf a
$ hg init a
$ cd a
$ echo a > a
$ hg ci -Am0
adding a
$ hg tag t1 # 1
$ hg tag --remove t1 # 2
$ hg co 1
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg tag -f -r0 t1
$ hg tags
tip 3:a49829c4fc11
t1 0:f7b1eb17ad24
$ cd ..