In case it helps anyone, I like to advance my Eclipse's cautiously trying new versions with experimental workspaces before I fully adopt a new version.
I, too, was having problems with the latest Eclipse and 2.7.2 MercurialEclipse.
I seen that some people are having trouble with Mercurial Eclipse on versions past Photon(4.8.0).
To Install:
Do the
This can get you past not being able to search Eclipse Marketplace.
You can select a version in the dialog box.
My Photon is working with MercurialEclipse 2.7.2.201905081900
For my Eclipse 2019-06 (4.12.0), I found that version 2.5.1.201804022240 works.
If you already installed the later version, you would have to uninstall that first.