diff_file: properly refcount blobs when initializing file contents

When initializing a `git_diff_file_content` from a source whose data is
derived from a blob, we simply assign the blob's pointer to the
resulting struct without incrementing its refcount. Thus, the structure
can only be used as long as the blob is kept alive by the caller.

Fix the issue by using `git_blob_dup` instead of a direct assignment.
This function will increment the refcount of the blob without allocating
new memory, so it does exactly what we want. As
`git_diff_file_content__unload` already frees the blob when
`GIT_DIFF_FLAG__FREE_BLOB` is set, we don't need to add new code
handling the free but only have to set that flag correctly.

src/diff_file.c

@@ -138,7 +138,6 @@ int git_diff_file_content__init_from_src(
 	memset(fc, 0, sizeof(*fc));
 	fc->repo = repo;
 	fc->file = as_file;
-	fc->blob = src->blob;
 
 	if (!src->blob && !src->buf) {
 		fc->flags |= GIT_DIFF_FLAG__NO_DATA;
@@ -148,12 +147,15 @@ int git_diff_file_content__init_from_src(
 		fc->file->mode = GIT_FILEMODE_BLOB;
 
 		if (src->blob) {
+			git_blob_dup((git_blob **)&fc->blob, (git_blob *) src->blob);
 			fc->file->size = git_blob_rawsize(src->blob);
 			git_oid_cpy(&fc->file->id, git_blob_id(src->blob));
 			fc->file->id_abbrev = GIT_OID_HEXSZ;
 
 			fc->map.len  = (size_t)fc->file->size;
 			fc->map.data = (char *)git_blob_rawcontent(src->blob);
+
+			fc->flags |= GIT_DIFF_FLAG__FREE_BLOB;
 		} else {
 			fc->file->size = src->buflen;
 			git_odb_hash(&fc->file->id, src->buf, src->buflen, GIT_OBJ_BLOB);

tests/diff/blob.c

 #include "clar_libgit2.h"
 #include "diff_helpers.h"
 
+#define BLOB_DIFF \
+    "diff --git a/file b/file\n" \
+    "index 45141a7..4d713dc 100644\n" \
+    "--- a/file\n" \
+    "+++ b/file\n" \
+    "@@ -1 +1,6 @@\n" \
+    " Hello from the root\n" \
+    "+\n" \
+    "+Some additional lines\n" \
+    "+\n" \
+    "+Down here below\n" \
+    "+\n"
+
 static git_repository *g_repo = NULL;
 static diff_expects expected;
 static git_diff_options opts;
@@ -65,6 +78,32 @@ static void assert_one_modified(
 	cl_assert_equal_i(dels,  exp->line_dels);
 }
 
+void test_diff_blob__patch_with_freed_blobs(void)
+{
+	git_oid a_oid, b_oid;
+	git_blob *a, *b;
+	git_patch *p;
+	git_buf buf = GIT_BUF_INIT;
+
+	/* tests/resources/attr/root_test1 */
+	cl_git_pass(git_oid_fromstrn(&a_oid, "45141a79", 8));
+	cl_git_pass(git_blob_lookup_prefix(&a, g_repo, &a_oid, 4));
+	/* tests/resources/attr/root_test2 */
+	cl_git_pass(git_oid_fromstrn(&b_oid, "4d713dc4", 8));
+	cl_git_pass(git_blob_lookup_prefix(&b, g_repo, &b_oid, 4));
+
+	cl_git_pass(git_patch_from_blobs(&p, a, NULL, b, NULL, NULL));
+
+	git_blob_free(a);
+	git_blob_free(b);
+
+	cl_git_pass(git_patch_to_buf(&buf, p));
+	cl_assert_equal_s(buf.ptr, BLOB_DIFF);
+
+	git_patch_free(p);
+	git_buf_free(&buf);
+}
+
 void test_diff_blob__can_compare_text_blobs(void)
 {
 	git_blob *a, *b, *c;