merge-recursive: combine error handling

merge-recursive: combine error handling

In handle_rename_rename_1to2(), we have duplicated error handling
around colliding paths. Specifically, when we want to write out
the file and there is a directory or untracked file in the way,
we need to create a temporary file to hold the contents. This has
some special output to alert the user, and this output is
duplicated for each side of the conflict.

Simplify the call by generating this new path in a helper
function.

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