revset: include the correct first ancestor change for follow(file)
Previously we always included '.', which may not touch a file.
Instead, find the file revision present in '.' and add its linkrev.
This matches the results of 'hg log --follow file'.
revsets: provide contexts for filesets
Before this change, revsets containing fileset patterns failed. This
allows queries like:
hg log -r "contains('set: added() and symlink()')"
Added signature for changeset
db33555eafea