merge: add a config option to disable path conflict checking

We've found a severe perf regression in `hg update` caused by the path conflict
checking code. The next patch will disable this by default.

Differential Revision: https://phab.mercurial-scm.org/D1222

mercurial/configitems.py

@@ -578,6 +578,9 coreconfigitem('merge', 'checkunknown',
 coreconfigitem('merge', 'checkignored',
     default='abort',
 )
+coreconfigitem('experimental', 'merge.checkpathconflicts',
+    default=True,
+)
 coreconfigitem('merge', 'followcopies',
     default=True,
 )

mercurial/merge.py

@@ -693,6 +693,7 def _checkunknownfiles(repo, wctx, mctx,
     abortconflicts = set()
     unknownconfig = _getcheckunknownconfig(repo, 'merge', 'checkunknown')
     ignoredconfig = _getcheckunknownconfig(repo, 'merge', 'checkignored')
+    pathconfig = repo.ui.configbool('experimental', 'merge.checkpathconflicts')
     if not force:
         def collectconflicts(conflicts, config):
             if config == 'abort':
@@ -704,7 +705,7 def _checkunknownfiles(repo, wctx, mctx,
             if m in ('c', 'dc'):
                 if _checkunknownfile(repo, wctx, mctx, f):
                     fileconflicts.add(f)
-                elif f not in wctx:
+                elif pathconfig and f not in wctx:
                     path = _checkunknowndirs(repo, f)
                     if path is not None:
                         pathconflicts.add(path)
@@ -1139,6 +1140,7 def manifestmerge(repo, wctx, p2, pa, br
                     actions[f] = ('dc', (None, f, f, False, pa.node()),
                                   "prompt deleted/changed")
 
+    if repo.ui.configbool('experimental', 'merge.checkpathconflicts'):
     # If we are merging, look for path conflicts.
     checkpathconflicts(repo, wctx, p2, actions)