rename detection limit checking, cherry picking, and git am -3

Linus' recent patch to invoke limiting on rename detection broke my ability to use cherry-picking on one project. This project has about 4300 files on one branch (a), 2500 on a later branch (b), 226 commits in total between the two branches, and a convoluted history of how branch a morphed into branch b. About 50 files were renamed in the transition, and we need to migrate patches from the still maintained branch a onto the new branch b.

Prior to Linus' recent patch to limit rename detection (0024a549), cherry picking a patch from a to b, where the patch affected just one file, often took about 45 seconds on a 3 GHz pentium 4 with the CPU pegged at 100% for the duration. The cherry picking always succeeded and correctly followed renames, but was very slow.

Following Linus' patch, the cherry picking fails with a merge conflict (almost instantly), complaining the file has been deleted on b but modified on a, i.e., the rename detection does not work. I tried raising diff.renameLimit to 100000, that seems to have no effect whatsoever on cherry-pick (the process aborts with a conflict almost immediately).

Curiously, using "git format-patch x..y --stdout | git am -3" succeeds in this case, and runs in well less than a second. This performance seems unchanged by the rename detection limit patch.

So, the rename limit patch "broke" git for this usage, though one could reasonably argue the previous code was so slow as to be broken anyway.

The curious thing to me is the vast superiority of whatever git-format-patch|git-am -3 does, and I wonder if that isn't a fundementally better design for cherry picking than git-cherry-pick implements (it obviously is for this case).

Mark