convert: return early when not tracing

convert: return early when not tracing

When Git adds a file requiring encoding conversion and tracing of encoding
conversion is not requested via the GIT_TRACE_WORKING_TREE_ENCODING
environment variable, the `trace_encoding()` function still allocates &
prepares "human readable" copies of the file contents before and after
conversion to show in the trace. This results in a high memory footprint
and increased runtime without providing any user-visible benefit.

This fix introduces an early exit from the `trace_encoding()` function
when tracing is not requested, preventing unnecessary memory allocation
and processing.

Signed-off-by: D Harithamma <harithamma.d@ibm.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>