t6042: add tests for consistency in file collision conflict handling

t6042: add tests for consistency in file collision conflict handling

Add testcases dealing with file collisions for the following types of
conflicts:
  * add/add
  * rename/add
  * rename/rename(2to1)

All these conflict types simplify down to two files "colliding"
and should thus be handled similarly.  This means that rename/add and
rename/rename(2to1) conflicts need to be modified to behave the same as
add/add conflicts currently do: the colliding files should be two-way
merged (instead of the current behavior of writing the two colliding
files out to separate temporary unique pathnames).  Add testcases which
check this; subsequent commits will fix the conflict handling to make
these tests pass.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>