.hgignore itself is version controlled. I've added some object dirs i want to ignore but i do not want to commit these changes to .hgignore. What do i do?
Add the following to the repo's .hg/hgrc:
[ui]
ignore = /path/to/repo/.hg/hgignore
and create a new file .hg/hgignore beside it. This new file will be untracked, but work the same as the versioned .hgignore file for this specific working copy. (The /path/to/repo bit is unfortunate but necessary to make it work when invoking hg from within a subdir of the repo.)
Source: Mercurial Tips and Tricks