Updates for corner cases in merge-recursive.
* en/merge-path-collision:
t6036: avoid non-portable "cp -a"
merge-recursive: combine error handling
t6036, t6043: increase code coverage for file collision handling
merge-recursive: improve rename/rename(1to2)/add[/add] handling
merge-recursive: use handle_file_collision for add/add conflicts
merge-recursive: improve handling for rename/rename(2to1) conflicts
merge-recursive: fix rename/add conflict handling
merge-recursive: new function for better colliding conflict resolutions
merge-recursive: increase marker length with depth of recursion
t6036, t6042: testcases for rename collision of already conflicting files
t6042: add tests for consistency in file collision conflict handling