maint.mk: avoid spurious failure of _sc_search_regexp-using tests * top/maint.mk: Initialize _sc_search_regexp parameters, so that envvar settings cannot interfere. Otherwise, setting envvars like prohibit=foo require=bar, etc. would cause spurious test failures.

# See lib/git-merge-changelog.c for making this useful.
ChangeLog	merge=merge-changelog

# Run this to make 'git diff' on texinfo files give nicer hunk context:
#   git config diff.texinfo.funcname '^@node[	 ][	 ]*\\([^,][^,]*\\)'
*.texi* diff=texinfo