git: better handling of absolute paths, regex metachars

git: better handling of absolute paths, regex metachars

Git currently gets a bit confused if you try to
manipulate files by absolute path.  There were also a
number of places where user-controlled file paths ended
up getting passed to regex interpretation, which could
confuse things.

This change mainly does 2 things:

	- Adds a 'drop' function which drops
	  a non-regex prefix from a string, and uses
	  that to manipulate paths, simplifies 'subst',
	  and removes 'subst -g', which was only used
	  with fixed regexes; sed does this job fine.
	- When getting a path from a user, we
	  make it absolute and then strip out the head

Along the way it cleans up a couple of stupids:

	- 'for(f in $list) if(! ~ $#f 0) use $f:
	  $f can't be a nil list because of
	  list flattening.
	- removes a useless substitution here:

	 	all=`$nl{{git/query -c $1 $2; git/query -c $2 $3} | sed 's/^..//' | \
			gsubst '^('$ourbr'|'$basebr'|'$theirbr')/*' | sort | uniq}

	  where git/query -c doesn't produce
	  paths prefixed with the query.