diffcore_rename(): use a stable sort

During Git's rename detection, the file names are sorted. At the moment,
this job is performed by `qsort()`. As that function is not guaranteed
to implement a stable sort algorithm, this can lead to inconsistent
and/or surprising behavior: a rename might be detected differently
depending on the platform where Git was run.

The `qsort()` in MS Visual C's runtime does _not_ implement a stable
sort algorithm, and it even leads to an inconsistency leading to a test
failure in t3030.35 "merge-recursive remembers the names of all base
trees": a different code path than on Linux is taken in the rename
detection of an ambiguous rename between either `e` to `a` or
`a~Temporary merge branch 2_0` to `a` during a recursive merge,
unexpectedly resulting in a clean merge.

Let's use the stable sort provided by `git_stable_qsort()` to avoid this
inconsistency.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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