fix: write the current answer into an entry the ledger forgot

A desktop entry that was edited in place is left alone once it carries the
marker, and the only thing that ever writes it again is the undo a change of
flags goes through - which reads the ledger. So an entry the ledger has lost is
one nothing looks at any more: it keeps whatever an older version put on its
command line, and an update that moves browsers to a different flag moves every
browser but that one.

The copy taken before the edit is named after the path, not recorded in the
ledger, and is still there. A marked entry with no ledger line is now put back
from that copy and written again from scratch, which is the same thing the undo
would have done, and it lands in the ledger on the way out.

Only entries edited in place need this. A generated entry is built from the
source entry every time and is right by construction, and a flag file has its
block rewritten whenever the contents differ.

src/lib/common.sh

@@ -293,6 +293,16 @@ mca_ledger_add() {
 	printf '%s\t%s\t%s\n' "$kind" "$path" "$detail" >> "$MCA_LEDGER"
 }
 
+# mca_ledger_has <path>
+# Whether the ledger still knows about that file. Worth asking because the copy
+# kept beside it is named after the path and outlives a ledger that was lost or
+# truncated: a file nothing has a record of any more is one nothing would ever
+# look at again.
+mca_ledger_has() {
+	[[ -f $MCA_LEDGER ]] || return 1
+	awk -F'\t' -v p="$1" '$2 == p { found = 1 } END { exit !found }' "$MCA_LEDGER"
+}
+
 mca_ledger_forget() {
 	local path="$1" tmp
 	[[ -f $MCA_LEDGER ]] || return 0