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.

sys/lib/git/common.rc

@@ -11,23 +11,22 @@ fn usage{
 	exit 'usage'
 }
 
-# subst [-g] this [that]
-fn subst{
-	awk 'BEGIN{
-		global = 0
-		for(i = 1; ARGV[i] ~ /^-/; i++){
-			if(ARGV[i] == "-g")
-				global = 1
-			ARGC--
-		}
-		this = ARGV[i++]; ARGC--
-		that = ARGV[i++]; ARGC--
-	}
+fn subst {
+	awk '
+	BEGIN{ARGC=0}
+	{sub(ARGV[1], ARGV[2]); print}
+	' $*
+}
+
+fn drop {
+	awk '
+	BEGIN{ARGC=0}
 	{
-		if(global) gsub(this, that)
-		else sub(this, that)
+		if(index($0, ARGV[1]) == 1)
+			$0=substr($0, length(ARGV[1])+1)
 		print
-	}' $*
+	}
+	' $*
 }
 
 fn present {
@@ -77,7 +76,7 @@ fn gitup{
 	if(~ $#gitroot 0)
 		die 'not a git repository'
 	gitfs=$gitroot/.git/fs
-	gitrel=`{pwd | subst '^'$"gitroot'/?'}
+	gitrel=`{pwd | drop $gitroot | sed 's@^/@@'}
 	if(~ $#gitrel 0)
 		gitrel='.'
 	cd $gitroot