am: avoid directory rename detection when calling recursive merge machinery

Let's say you have the following three trees, where Base is from one commit
behind either master or branch:

Base  : bar_v1, foo/{file1, file2, file3}
branch: bar_v2, foo/{file1, file2},       goo/file3
master: bar_v3, foo/{file1, file2, file3}

Using git-am (or am-based rebase) to apply the changes from branch onto
master results in the following tree:

Result: bar_merged, goo/{file1, file2, file3}

This is not what users want; they did not rename foo/ -> goo/, they only
renamed one file within that directory.  The reason this happens is am
constructs fake trees (via build_fake_ancestor()) of the following form:

Base_bfa  : bar_v1, foo/file3
branch_bfa: bar_v2, goo/file3

Combining these two trees with master's tree:

master: bar_v3, foo/{file1, file2, file3},

You can see that merge_recursive_generic() would see branch_bfa as renaming
foo/ -> goo/, and master as just adding both foo/file1 and foo/file2.  As
such, it ends up with goo/{file1, file2, file3}

The core problem is that am does not have access to the original trees; it
can only construct trees using the blobs involved in the patch.  As such,
it is not safe to perform directory rename detection within am -3.

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