git-gui: don't delete source files when auto_mkindex fails

Commit 2cc5b0facfa4 (git-gui: extract script to generate "tclIndex", 2025-03-11) converted commands in a Makefile rule to a shell script. In this process, the Makefile variable $@ had to be replaced by the file name that it represents, 'lib/tclIndex'. However, the occurrence in `rm -f $@` was missed. In a shell script, $@ expands to all command line arguments, which happen to be the source files lib/*.tcl in this case. Needless to say that we do not want to remove source files during a build. Replace $@ by the intended 'lib/tclIndex'. Reported-by: Randall S. Becker <rsbecker@nexbridge.com> Signed-off-by: Johannes Sixt <j6t@kdbg.org>