Fix unindented comments being corrupted in indented blocks

When an unindented comment appears before an indented import block,
isort blindly strips `len(indent)` characters from the start of every
line. This corrupts comments that don't start with the expected indent.

For example:
```python
if True:
# this will get cut off
    import os
```
became:
```python
if True:
is will get cut off
    import os
```

The fix checks whether each line actually starts with the indent
before stripping it.

Fixes #1899

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>