Render tabs with 8 spaces (previously 4)

Most web UIs seem to do this (e.g. on GitHub), and C code from the
Linux kernel assumes 8 spaces in a tab when aligning things.

CHANGELOG.md

@@ -17,6 +17,8 @@ Improved performance in large files when changes are clustered together.
 
 Improved syntax highlighting for keywords.
 
+Tabs are now rendered with eight spaces.
+
 ### Command Line Interface
 
 Difftastic now validates environment variables the same way as

src/main.rs

@@ -246,10 +246,10 @@ fn diff_file_content(
     // TODO: don't replace tab characters inside string literals.
     let mut lhs_src = String::from_utf8_lossy(lhs_bytes)
         .to_string()
-        .replace('\t', "    ");
+        .replace('\t', "        ");
     let mut rhs_src = String::from_utf8_lossy(rhs_bytes)
         .to_string()
-        .replace('\t', "    ");
+        .replace('\t', "        ");
 
     // Ignore the trailing newline, if present.
     // TODO: highlight if this has changes (#144).