Merge branch 'tb/config-fixed-value-with-valueless-true' into maint-2.46

"git config --value=foo --fixed-value section.key newvalue" barfed when the existing value in the configuration file used the valueless true syntax, which has been corrected. * tb/config-fixed-value-with-valueless-true: config.c: avoid segfault with --fixed-value and valueless config