apply: when -R, also reverse list of sections

apply: when -R, also reverse list of sections

A patch changing a symlink into a file is written with 2 sections (in
the code, represented as "struct patch"): firstly, the deletion of the
symlink, and secondly, the creation of the file. When applying that
patch with -R, the sections are reversed, so we get:

 (1) creation of a symlink, then
 (2) deletion of a file.

This causes an issue when the "deletion of a file" section is checked,
because Git observes that the so-called file is not a file but a
symlink, resulting in a "wrong type" error message.

What we want is:

 (1) deletion of a file, then
 (2) creation of a symlink.

In the code, this is reflected in the behavior of previous_patch() when
invoked from check_preimage() when the deletion is checked. Creation
then deletion means that when the deletion is checked, previous_patch()
returns the creation section, triggering a mode conflict resulting in
the "wrong type" error message. But deletion then creation means that
when the deletion is checked, previous_patch() returns NULL, so the
deletion mode is checked against lstat, which is what we want.

There are also other ways a patch can contain 2 sections referencing the
same file, for example, in 7a07841c0b ("git-apply: handle a patch that
touches the same path more than once better", 2008-06-27). "git apply
-R" fails in the same way, and this commit makes this case succeed.

Therefore, when building the list of sections, build them in reverse
order (by adding to the front of the list instead of the back) when -R
is passed.

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>