Fix UnboundLocalError in nested config updates

[krantboy]

Fixes #10348

ConfigFileWriter._update_subattributes() reads current_indent on every line
but only assigns it when OPTION_REGEX matches, so any non-option line directly
after a nested key is read before assignment. i has the same problem when the
nested key is the last line in the file and the loop body never runs.

All three of these shapes crash on aws configure set s3.signature_version s3v4:

[default]        [default]        [default]
s3 =             s3 =             s3 =
# comment        [profile foo]    (end of file)

UnboundLocalError: cannot access local variable 'current_indent'
where it is not associated with a value

The fix classifies each line explicitly instead of carrying the previous match's
indent forward, and seeds i before the loop. It also adds a missing trailing
newline before appending, without which the above would turn the crash into a
corrupted config file.

Tests cover all three shapes plus an empty nested block with no trailing
newline; each fails before the change.