Add read-only option for hgrc

Created originally on Bitbucket by tw_bert (Tijs Bertus)

In our company, on various occasions confusion has arisen because the hgrc file was corrupted.

As it turned out, MercurialEclipse rewrites the hgrc file (when editing Project Properties), without warning.

Our hgrc files almost always consist of only 1 line, pointing to a versioned one (here a windows-based one, but we also have plenty unix-based ones). Example:

%include %MainEclipseWorkspace%hgrc_versioning\sharedconfig\hgrc_shared_win__xxxx_documentation.ini

MercurialEclipse should stay clear of our hgrc files.

We actually had to make an ad-hoc service around it, to be able to backup and restore hgrc files that are polluted by MercurialEclipse. We have dozens of repositories, and dozens of developers/identities.

Please add an option (in Eclipse preferences and/or in hg settings) to steer MercurialEclipse away from modifying a hgrc file.

Sideote: We still love MercurialEclipse of course.