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

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>