dim: Explain how to resolve conflicts when git rerere fails

Chris ran into this, and I guess worth documenting best practices
since it happens sooooooo rarely.

v2: Clarify stuff a bit.

v3: Also normalize branch names in conflict markers. Without this we
can't use fixup patches to fixup up conflicts when git rerere fails to
store/reapply the resolution.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>