Fix multibyte codepoint crash (#1243)

Co-authored-by: Evan Richter <[email protected]>

src/unindent.rs

@@ -5,7 +5,7 @@ pub fn unindent(text: &str) -> String {
   let mut start = 0;
   for (i, c) in text.char_indices() {
     if c == '\n' || i == text.len() - c.len_utf8() {
-      let end = i + 1;
+      let end = i + c.len_utf8();
       lines.push(&text[start..end]);
       start = end;
     }

tests/lib.rs

@@ -57,6 +57,7 @@ mod invocation_directory;
 mod json;
 mod line_prefixes;
 mod misc;
+mod multibyte_char;
 mod positional_arguments;
 mod quiet;
 mod quote;

tests/multibyte_char.rs

@@ -0,0 +1,6 @@
+use super::*;
+
+#[test]
+fn bugfix() {
+  Test::new().justfile("foo:\nx := '''ǩ'''").run();
+}