tweakdefaults: make hg resolve require --re-merge flag to re-merge

Pulkit suggested it in https://phab.mercurial-scm.org/D4379, and a
discussion with Octobus people reminded me that people still use the
error-prone default behavior of `hg resolve`.

Differential Revision: https://phab.mercurial-scm.org/D6610

mercurial/ui.py

@@ -68,6 +68,8 b' grep.all-files = True'
 update.check = noconflict
 # Show conflicts information in `hg status`
 status.verbose = True
+# Make `hg resolve` with no action (like `-m`) fail instead of re-merging.
+resolve.explicit-re-merge = True
 
 [diff]
 git = 1