Add documentation on git bisecting across the MLIR merge

Differential Revision: https://reviews.llvm.org/D73988

docs/GitBisecting.rst

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+===================
+Bisecting LLVM code
+===================
+
+Introduction
+============
+
+``git bisect`` is a useful tool for finding which revision caused a bug.
+
+This document describes how to use ``git bisect``. In particular, while LLVM
+has a mostly linear history, it has a few merge commits that added projects --
+and these merged the linear history of those projects. As a consequence, the
+LLVM repository has multiple roots: One "normal" root, and then one for each
+toplevel project that was developed out-of-tree and then merged later.
+As of early 2020, the only such merged project is MLIR, but flang will likely
+be merged in a similar way soon.
+
+Basic operation
+===============
+
+See https://git-scm.com/docs/git-bisect for a good overview. In summary:
+
+  .. code-block:: bash
+
+     git bisect start
+     git bisect bad master
+     git bisect good f00ba
+
+git will check out a revision in between. Try to reproduce your problem at
+that revision, and run ``git bisect good`` or ``git bisect bad``.
+
+If you can't repro at the current commit (maybe the build is broken), run
+``git bisect skip`` and git will pick a nearby alternate commit.
+
+(To abort a bisect, run ``git bisect reset``, and if git complains about not
+being able to reset, do the usual ``git checkout -f master; git reset --hard
+origin/master`` dance and try again).
+
+``git bisect run``
+==================
+
+A single bisect step often requires first building clang, and then compiling
+a large code base with just-built clang. This can take a long time, so it's
+good if it can happen completely automatically. ``git bisect run`` can do
+this for you if you write a run script that reproduces the problem
+automatically. Writing the script can take 10-20 minutes, but it's almost
+always worth it -- you can do something else while the bisect runs (such
+as writing this document).
+
+Here's an example run script. It assumes that you're in ``llvm-project`` and
+that you have a sibling ``llvm-build-project`` build directory where you
+configured CMake to use Ninja. You have a file ``repro.c`` in the current
+directory that makes clang crash at trunk, but it worked fine at revision
+``f00ba``.
+
+  .. code-block:: bash
+
+     # Build clang. If the build fails, `exit 125` causes this
+     # revision to be skipped
+     ninja -C ../llvm-build-project clang || exit 125
+
+     ../llvm-build-project/bin/clang repro.c
+
+To make sure your run script works, it's a good idea to run ``./run.sh`` by
+hand and tweak the script until it works, then run ``git bisect good`` or
+``git bisect bad`` manually once based on the result of the script 
+(check ``echo $?`` after your script ran), and only then run ``git bisect run
+./run.sh``. Don't forget to mark your run script as executable -- ``git bisect
+run`` doesn't check for that, it just assumes the run script failed each time.
+
+Once your run script works, run ``git bisect run ./run.sh`` and a few hours
+later you'll know which commit caused the regression.
+
+(This is a very simple run script. Often, you want to use just-built clang
+to build a different project and then run a built executable of that project
+in the run script.)
+
+Bisecting across multiple roots
+===============================
+
+Here's how LLVM's history currently looks:
+
+  .. code-block::
+
+     A-o-o-......-o-D-o-o-HEAD
+                   /
+       B-o-...-o-C-
+ 
+``A`` is the first commit in LLVM ever, ``97724f18c79c``.
+
+``B`` is the first commit in MLIR, ``aed0d21a62db``.  ``D`` is the merge commit
+that merged MLIR into the main LLVM repository, ``0f0d0ed1c78f``. ``C`` is the
+last commit in MLIR before it got merged, ``0f0d0ed1c78f^2``. (The ``^n``
+modifier selects the n'th parent of a merge commit.)
+
+``git bisect`` goes through all parent revisions. Due to the way MLIR was
+merged, at every revision at ``C`` or earlier, *only* the ``mlir/`` directory
+exists, and nothing else does.
+
+As of early 2020, there is no flag to ``git bisect`` to tell it to not
+descend into all reachable commits. Ideally, we'd want to tell it to only
+follow the first parent of ``D``.
+
+The best workaround is to pass a list of directories to ``git bisect``:
+If you know the bug is due to a change in llvm, clang, or compiler-rt, use
+``git bisect start -- clang llvm compiler-rt``. That way, the commits in
+``mlir`` are never evaluated.
+
+Alternatively, ``git bisect skip aed0d21a6 aed0d21a6..0f0d0ed1c78f`` explicitly
+skips all commits on that branch. It takes 1.5 minutes to run on a fast
+machine, and makes ``git bisect log`` output unreadable. (``aed0d21a6`` is
+listed twice because git ranges exclude the revision listed on the left,
+so it needs to be ignored explicitly.)
+
+More Resources
+==============
+
+https://git-scm.com/book/en/v2/Git-Tools-Revision-Selection