Edited 10th June '11 - use update site: http://download.eclipse.org/egit/updates
There is an issue at the moment when you try and install the egit/jgit plugins with Indigo versions RC3 and above.
You'll encounter an error similar to this:
An error occurred while collecting items to be installed session context was:(profile=epp.package.rcp, phase=org.eclipse.equinox.internal.p2.engine.phases.Collect, operand=, action=). No repository found containing: osgi.bundle,org.eclipse.egit,1.0.0.201106011211-rc3 No repository found containing: osgi.bundle,org.eclipse.egit.core,1.0.0.201106011211-rc3 No repository found containing: osgi.bundle,org.eclipse.egit.doc,1.0.0.201106011211-rc3 No repository found containing: osgi.bundle,org.eclipse.egit.ui,1.0.0.201106011211-rc3 No repository found containing: osgi.bundle,org.eclipse.jgit,1.0.0.201106011211-rc3
This is caused by the latest jgit packages not yet being in the indigo release p2 update repository. To get these installed until these updates are provided, add http://download.eclipse.org/egit/staging/ as an update site and you'll be able to install these plugins successfully.
I found this solution here: https://bugs.eclipse.org/bugs/show_bug.cgi?id=348183