refactor(toml): Preserve the full newline for shebang

The shebang is thrown away so this has no end-user impact

Author: epage

src/cargo/util/toml/embedded.rs

@@ -216,10 +216,12 @@ fn split_source(input: &str) -> CargoResult<Source<'_>> {
         }
 
         // No other choice than to consider this a shebang.
-        let (shebang, content) = source
+        let newline_end = source
             .content
-            .split_once('\n')
-            .unwrap_or((source.content, ""));
+            .find('\n')
+            .map(|pos| pos + 1)
+            .unwrap_or(source.content.len());
+        let (shebang, content) = source.content.split_at(newline_end);
         source.shebang = Some(shebang);
         source.content = content;
     }
@@ -401,7 +403,7 @@ fn main() {}
 Ok(
     Source {
         shebang: Some(
-            "#!/usr/bin/env cargo",
+            "#!/usr/bin/env cargo\n",
         ),
         info: None,
         frontmatter: Some(
@@ -427,7 +429,7 @@ Ok(
 Ok(
     Source {
         shebang: Some(
-            "#!/usr/bin/env cargo\r",
+            "#!/usr/bin/env cargo\r\n",
         ),
         info: Some(
             "",
@@ -466,7 +468,7 @@ fn main() {}
 Ok(
     Source {
         shebang: Some(
-            "#!/usr/bin/env cargo",
+            "#!/usr/bin/env cargo\n",
         ),
         info: None,
         frontmatter: None,
@@ -524,7 +526,7 @@ fn main() {}"#
 Ok(
     Source {
         shebang: Some(
-            "#!/usr/bin/env cargo",
+            "#!/usr/bin/env cargo\n",
         ),
         info: None,
         frontmatter: Some(