str: introduce git_str_shellquote

Introduce a helper method to quote a string in a shellsafe manner.  This
wraps the entire buffer in single quotes, escaping single-quotes and
exclamation points.

src/util/str.c

@@ -1415,3 +1415,40 @@ done:
 	git_str_dispose(&replaced);
 	return error;
 }
+
+int git_str_shellquote(git_str *buf)
+{
+	git_str quoted = GIT_STR_INIT;
+	size_t i;
+	int error = 0;
+
+	ENSURE_SIZE(&quoted, buf->size);
+
+	git_str_putc(&quoted, '\'');
+
+	for (i = 0; i < buf->size; i++) {
+		switch (buf->ptr[i]) {
+		case '\'':
+		case '!':
+			git_str_puts(&quoted, "'\\");
+			git_str_putc(&quoted, buf->ptr[i]);
+			git_str_putc(&quoted, '\'');
+			break;
+		default:
+			git_str_putc(&quoted, buf->ptr[i]);
+		}
+	}
+
+	git_str_putc(&quoted, '\'');
+
+	if (git_str_oom(&quoted)) {
+		error = -1;
+		goto done;
+	}
+
+	git_str_swap(&quoted, buf);
+
+done:
+	git_str_dispose(&quoted);
+	return error;
+}

src/util/str.h

@@ -366,4 +366,10 @@ int git_str_replace(
 	const char *replacements[][2],
 	size_t replacements_len);
 
+/**
+ * Quote for shell safety.  Wrap the given buffer in single quotes,
+ * escaping any single quotes and exclamation points.
+ */
+int git_str_shellquote(git_str *buf);
+
 #endif

tests/util/str/basic.c

@@ -64,3 +64,32 @@ void test_str_basic__replace(void)
 
 	git_str_dispose(&buf);
 }
+
+void test_str_basic__shellquote(void)
+{
+	git_str buf = GIT_BUF_INIT;
+
+	cl_git_pass(git_str_puts(&buf, "filename"));
+	cl_git_pass(git_str_shellquote(&buf));
+	cl_assert_equal_s("\'filename\'", buf.ptr);
+
+	git_str_clear(&buf);
+
+	cl_git_pass(git_str_puts(&buf, "file name"));
+	cl_git_pass(git_str_shellquote(&buf));
+	cl_assert_equal_s("\'file name\'", buf.ptr);
+
+	git_str_clear(&buf);
+
+	cl_git_pass(git_str_puts(&buf, "file\'name"));
+	cl_git_pass(git_str_shellquote(&buf));
+	cl_assert_equal_s("\'file\'\\\'\'name\'", buf.ptr);
+
+	git_str_clear(&buf);
+
+	cl_git_pass(git_str_puts(&buf, "file!name"));
+	cl_git_pass(git_str_shellquote(&buf));
+	cl_assert_equal_s("\'file\'\\!\'name\'", buf.ptr);
+
+	git_str_dispose(&buf);
+}