merge-tools: when calling external merge tool, describe the resolve inputs
It is a common complaint that a user will be running some operation (histedit,
rebase, evolve, etc.), get into a merge-conflict situation, and not understand
what they are seeing - it is possible that the merge tool is configured to
display the hash, but it's difficult for most merge tools to display a good
snippet of the description.
In the worst case, configuring this template will lead to output that is
immediately covered by a terminal application, maybe the user can hit ctrl-z to
see it. In the common case, the output will be in a terminal window and a GUI
program will start, and it should be possible to view both the terminal and the
GUI program at the same time.
Differential Revision: https://phab.mercurial-scm.org/D5094