rebase: disable `inmemory` if the rebaseset contains the working copy

As described in the comment, rebasing the working copy parent with in-memory
merge, and then updating to the new commit, isn't much faster because of the
extra overhead of uppdating. Best to leave it off in that case.

This commit makes deploying in-memory merge via an extension easier, because
you can just set `inmemory=True` based on some config or probability, and this
will turn off the cases where it's not desired.

Differential Revision: https://phab.mercurial-scm.org/D1616

hgext/rebase.py

@@ -830,7 +830,8 b' def _origrebase(ui, repo, **opts):'
         else:
             destmap = _definedestmap(ui, repo, destf, srcf, basef, revf,
                                      destspace=destspace,
-                                     inmemory=opts['inmemory'])
+                                     opts=opts)
+            rbsrt.inmemory = opts['inmemory']
             retcode = rbsrt._preparenewrebase(destmap)
             if retcode is not None:
                 return retcode
@@ -850,7 +851,7 b' def _origrebase(ui, repo, **opts):'
         rbsrt._finishrebase()
 
 def _definedestmap(ui, repo, destf=None, srcf=None, basef=None, revf=None,
-                   destspace=None, inmemory=False):
+                   destspace=None, opts=None):
     """use revisions argument to define destmap {srcrev: destrev}"""
     if revf is None:
         revf = []
@@ -864,7 +865,7 b' def _definedestmap(ui, repo, destf=None,'
     if revf and srcf:
         raise error.Abort(_('cannot specify both a revision and a source'))
 
-    if not inmemory:
+    if not opts['inmemory']:
         cmdutil.checkunfinished(repo)
         cmdutil.bailifchanged(repo)
 
@@ -939,6 +940,22 b' def _definedestmap(ui, repo, destf=None,'
                 ui.status(_('nothing to rebase from %s to %s\n') %
                           ('+'.join(str(repo[r]) for r in base), dest))
             return None
+    # If rebasing the working copy parent, force in-memory merge to be off.
+    #
+    # This is because the extra work of checking out the newly rebased commit
+    # outweights the benefits of rebasing in-memory, and executing an extra
+    # update command adds a bit of overhead, so better to just do it on disk. In
+    # all other cases leave it on.
+    #
+    # Note that there are cases where this isn't true -- e.g., rebasing large
+    # stacks that include the WCP. However, I'm not yet sure where the cutoff
+    # is.
+    rebasingwcp = repo['.'].rev() in rebaseset
+    if opts['inmemory'] and rebasingwcp:
+        opts['inmemory'] = False
+        # Check these since we did not before.
+        cmdutil.checkunfinished(repo)
+        cmdutil.bailifchanged(repo)
 
     if not destf:
         dest = repo[_destrebase(repo, rebaseset, destspace=destspace)]