debug: add a config to abort update early

This is useful to benchmark the parts of [hg update] that come
before the parallel worker.

mercurial/configitems.py

@@ -746,6 +746,14 b' coreconfigitem('
     b'discovery.exchange-heads',
     default=True,
 )
+# If devel.debug.abort-update is True, then any merge with the working copy,
+# e.g. [hg update], will be aborted after figuring out what needs to be done,
+# but before spawning the parallel worker
+coreconfigitem(
+    b'devel',
+    b'debug.abort-update',
+    default=False,
+)
 # If discovery.grow-sample is False, the sample size used in set discovery will
 # not be increased through the process
 coreconfigitem(

mercurial/merge.py

@@ -1142,6 +1142,8 b' def calculateupdates('
             followcopies,
         )
         _checkunknownfiles(repo, wctx, mctx, force, mresult, mergeforce)
+        if repo.ui.configbool(b'devel', b'debug.abort-update'):
+            exit(1)
 
     else:  # only when merge.preferancestor=* - the default
         repo.ui.note(