apply: use an indexwriter

Place the entire `git_apply` operation inside an indexwriter, so that we
lock the index before we begin performing patch application.  This
ensures that there are no other processes modifying things in the
working directory.

src/apply.c

@@ -22,6 +22,7 @@
 #include "delta.h"
 #include "zstream.h"
 #include "reader.h"
+#include "index.h"
 
 #define apply_err(...) \
 	( giterr_set(GITERR_PATCH, __VA_ARGS__), GIT_EAPPLYFAIL )
@@ -556,6 +557,7 @@ static int git_apply__to_workdir(
 
 	checkout_opts.checkout_strategy |= GIT_CHECKOUT_SAFE;
 	checkout_opts.checkout_strategy |= GIT_CHECKOUT_DISABLE_PATHSPEC_MATCH;
+	checkout_opts.checkout_strategy |= GIT_CHECKOUT_DONT_WRITE_INDEX;
 
 	if (opts->location == GIT_APPLY_LOCATION_WORKDIR)
 		checkout_opts.checkout_strategy |= GIT_CHECKOUT_DONT_UPDATE_INDEX;
@@ -610,8 +612,6 @@ static int git_apply__to_index(
 			goto done;
 	}
 
-	error = git_index_write(index);
-
 done:
 	git_index_free(index);
 	return error;
@@ -638,7 +638,8 @@ int git_apply(
 	git_diff *diff,
 	git_apply_options *given_opts)
 {
-	git_index *preimage = NULL, *postimage = NULL;
+	git_indexwriter indexwriter = GIT_INDEXWRITER_INIT;
+	git_index *index = NULL, *preimage = NULL, *postimage = NULL;
 	git_reader *pre_reader = NULL;
 	git_apply_options opts = GIT_APPLY_OPTIONS_INIT;
 	size_t i;
@@ -685,6 +686,10 @@ int git_apply(
 	    (error = git_index_new(&postimage)) < 0)
 		goto done;
 
+	if ((error = git_repository_index(&index, repo)) < 0 ||
+	    (error = git_indexwriter_init(&indexwriter, index)) < 0)
+		goto done;
+
 	for (i = 0; i < git_diff_num_deltas(diff); i++) {
 		if ((error = apply_one(repo, pre_reader, preimage, postimage, diff, i)) < 0)
 			goto done;
@@ -707,9 +712,16 @@ int git_apply(
 	if (error < 0)
 		goto done;
 
+	if (error < 0)
+		goto done;
+
+	error = git_indexwriter_commit(&indexwriter);
+
 done:
+	git_indexwriter_cleanup(&indexwriter);
 	git_index_free(postimage);
 	git_index_free(preimage);
+	git_index_free(index);
 	git_reader_free(pre_reader);
 
 	return error;