am -3: allow nonstandard -p<num> option

When falling back to 3-way merge, we run "git apply" to synthesize the
fake ancestor tree by parsing the incoming patch, and another "git apply"
to apply the patch to the fake ancestor tree.  Both invocation need to
be aware of the custom -p<num> setting to parse patches that were prepared
with non-standard src/dst prefix.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

git-am.sh

@@ -127,15 +127,18 @@ fall_back_3way () {
     mkdir "$dotest/patch-merge-tmp-dir"
 
     # First see if the patch records the index info that we can use.
-    git apply --build-fake-ancestor "$dotest/patch-merge-tmp-index" \
-	"$dotest/patch" &&
+    cmd="git apply $git_apply_opt --build-fake-ancestor" &&
+    cmd="$cmd "'"$dotest/patch-merge-tmp-index" "$dotest/patch"' &&
+    eval "$cmd" &&
     GIT_INDEX_FILE="$dotest/patch-merge-tmp-index" \
     git write-tree >"$dotest/patch-merge-base+" ||
     cannot_fallback "$(gettext "Repository lacks necessary blobs to fall back on 3-way merge.")"
 
     say Using index info to reconstruct a base tree...
-    if GIT_INDEX_FILE="$dotest/patch-merge-tmp-index" \
-	git apply --cached <"$dotest/patch"
+
+    cmd='GIT_INDEX_FILE="$dotest/patch-merge-tmp-index"'
+    cmd="$cmd git apply --cached $git_apply_opt"' <"$dotest/patch"'
+    if eval "$cmd"
     then
 	mv "$dotest/patch-merge-base+" "$dotest/patch-merge-base"
 	mv "$dotest/patch-merge-tmp-index" "$dotest/patch-merge-index"