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

Commit 2cc5b0f (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 <[email protected]>
Signed-off-by: Johannes Sixt <[email protected]>

Author: j6t

generate-tclindex.sh

@@ -23,7 +23,7 @@ then
 	: ok
 else
 	 echo >&2 "    * $TCL_PATH failed; using unoptimized loading"
-	 rm -f $@
+	 rm -f lib/tclIndex
 	 echo '# Autogenerated by git-gui Makefile' >lib/tclIndex
 	 echo >>lib/tclIndex
 	 echo "class.tcl" >>lib/tclIndex