tests: apply: verify that we correctly truncate the source buffer

Previously, we would fail to correctly truncate the source buffer
if the source has more than one line and ends with a non-newline
character. In the following call, we thus truncate the source
string in the middle of the second line. Without the bug fixed,
we would successfully apply the patch to the source and return
success. With the overflow being fixed, we should return an
error now.

tests/apply/fromdiff.c

@@ -333,3 +333,36 @@ void test_apply_fromdiff__binary_delete(void)
 		NULL, NULL,
 		NULL, &binary_opts));
 }
+
+void test_apply_fromdiff__patching_correctly_truncates_source(void)
+{
+	git_buf original = GIT_BUF_INIT, patched = GIT_BUF_INIT;
+	git_patch *patch;
+	unsigned int mode;
+	char *path;
+
+	cl_git_pass(git_patch_from_buffers(&patch,
+					   "foo\nbar", 7, "file.txt",
+					   "foo\nfoo", 7, "file.txt", NULL));
+
+	/*
+	 * Previously, we would fail to correctly truncate the source buffer if
+	 * the source has more than one line and ends with a non-newline
+	 * character. In the following call, we thus truncate the source string
+	 * in the middle of the second line. Without the bug fixed, we would
+	 * successfully apply the patch to the source and return success. With
+	 * the overflow being fixed, we should return an error.
+	 */
+	cl_git_fail_with(GIT_EAPPLYFAIL,
+			 git_apply__patch(&patched, &path, &mode,
+					  "foo\nbar\n", 5, patch, NULL));
+
+	/* Verify that the patch succeeds if we do not truncate */
+	cl_git_pass(git_apply__patch(&patched, &path, &mode,
+				     "foo\nbar\n", 7, patch, NULL));
+
+	git_buf_dispose(&original);
+	git_buf_dispose(&patched);
+	git_patch_free(patch);
+	git__free(path);
+}