This is an unloved feature.
This extension is currently being distributed along with Mercurial.
Author: Vadim Gelfer
This extension is a convenience wrapper. In the simplest case, hg fetch acts like hg pull -u -- it pulls changes from a remote repository into a local one and updates the working directory. (This is like the "update" command as found in Subversion or CVS.)
If the pulled changes require merging, the fetch extension attempts an hg merge followed by an hg commit.
To enable the extension, add the following lines to your .hgrc:
hg fetch [SOURCE]
Where SOURCE is a path or URL.
Use hg help fetch for additional options.