csv: strip outer whitespace when interpolating CSV values (#1051)

This removes a potential snag in amount field assignments, and
hopefully is harmless and acceptable otherwise.

hledger-lib/Hledger/Read/CsvReader.hs

@@ -850,10 +850,12 @@ getEffectiveAssignment rules record f = lastMay $ assignmentsFor f
                       where
                         csvline = intercalate "," record
 
+-- | Render a field assigment's template, possibly interpolating referenced
+-- CSV field values. Outer whitespace is removed from interpolated values.
 renderTemplate ::  CsvRules -> CsvRecord -> FieldTemplate -> String
 renderTemplate rules record t = regexReplaceBy "%[A-z0-9]+" replace t
   where
-    replace ('%':pat) = maybe pat (\i -> atDef "" record (i-1)) mindex
+    replace ('%':pat) = maybe pat (\i -> strip $ atDef "" record (i-1)) mindex
       where
         mindex | all isDigit pat = readMay pat
                | otherwise       = lookup (map toLower pat) $ rcsvfieldindexes rules

hledger-lib/hledger_csv.m4.md

@@ -161,8 +161,12 @@ amount USD %4
 # combine three fields to make a comment (containing two tags)
 comment note: %somefield - %anotherfield, date: %1
 ```
+
 Field assignments can be used instead of or in addition to a field list.
 
+Note, interpolation strips any outer whitespace, so a CSV value like
+`" 1 "` becomes `1` when interpolated.
+
 ## conditional block
 
 `if` *`PATTERN`*\