diffcore-rename: remove unnecessary duplicate entry checks

Commit 25d5ea410f ("[PATCH] Redo rename/copy detection logic.",
2005-05-24) added a duplicate entry check on rename_src in order to
avoid segfaults; the code at the time was prone to double free()s and an
easy way to avoid it was just to turn off rename detection for any
duplicate entries.  Note that the form of the check was modified two
commits ago in this series.

Similarly, commit 4d6be03b95 ("diffcore-rename: avoid processing
duplicate destinations", 2015-02-26) added a duplicate entry check
on rename_dst for the exact same reason -- the code was prone to double
free()s, and an easy way to avoid it was just to turn off rename
detection entirely.  Note that the form of the check was modified in the
commit just before this one.

In the original code in both places, the code was dealing with
individual diff_filespecs and trying to match things up, instead of just
keeping the original diff_filepairs around as we do now.  The
intervening change in structure has fixed the accounting problems and
the associated double free()s that used to occur, and thus we already
have a better fix.  As such, we can remove the band-aid checks for
duplicate entries.

Due to the last two patches, the diffcore_rename() setup is no longer a
sizeable chunk of overall runtime.  Thus, in a large rebase of many
commits with lots of renames and several optimizations to inexact rename
detection, this patch only speeds up the overall code by about half a
percent or so and is pretty close to the run-to-run variability making
it hard to get an exact measurement.  However, with some trace2 regions
around the setup code in diffcore_rename() so that I can focus on just
it, I measure that this patch consistently saves almost a third of the
remaining time spent in diffcore_rename() setup.

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

t/t4058-diff-duplicates.sh

@@ -91,7 +91,7 @@ test_expect_success 'diff-tree between duplicate trees' '
 test_expect_success 'diff-tree with renames' '
 	# See NOTICE at top of file.
 	git diff-tree -M -r --no-abbrev one two >actual &&
-	test_cmp expect actual
+	test_must_be_empty actual
 '
 
 test_expect_success 'diff-tree FROM duplicate tree' '