filemerge: use a single temp dir instead of temp files This can help to remove the clutter from UIs that display just the filenames; instead of seeing foo~local.C9ru9r.txt and foo~base.2DMV22.txt (in the /tmp directory on most platforms), we create a single new directory and use that, producing filenames like /tmp/hgmerge.C9ru9r/foo~local.txt. Differential Revision: https://phab.mercurial-scm.org/D2888

--- a/mercurial/configitems.py	Fri Jan 19 19:14:09 2018 -0800
+++ b/mercurial/configitems.py	Fri Jan 19 19:07:58 2018 -0800
@@ -502,6 +502,9 @@
 coreconfigitem('experimental', 'maxdeltachainspan',
     default=-1,
 )
+coreconfigitem('experimental', 'mergetempdirprefix',
+    default=None,
+)
 coreconfigitem('experimental', 'mmapindexthreshold',
     default=None,
 )