Only leave space for git diff markers if any line is modified (#3406)

Previously, setting `--style=changes` would always print a 2-space
indent, even if the file was unmodified. This changes the style to only
print an indent if there is at least one +/- git marker in the sidebar.

CHANGELOG.md

@@ -20,6 +20,7 @@
 - Send all bat errors to stderr by default, see #3336 (@JerryImMouse)
 - Make --map-syntax target case insensitive to match --language, see #3206 (@keith-hall)
 - Correctly determine the end of the line in UTF16LE/BE input #3369 (@keith-hall)
+- `--style=changes` no longer prints a two-space indent when the file is unmodified, see issue #2710 and PR #3406 (@jyn514)
 
 ## Other