cli: use a blank acct name for postings without the specified pivot tag

Eg. if you --pivot code, postings from transactions with no code value
were formerly displayed with the unpivoted account name, so you'd see
a mixture of account names and codes. Now we always pivot the account name,
to blank if nothing else. This probably makes reports pretty ugly, it's just another
step towards making --pivot's behaviour clearer.

hledger/Hledger/Cli/Utils.hs

@@ -92,7 +92,7 @@ pivot tag j = j{jtxns = map pivotTrans . jtxns $ j}
   pivotTrans t = t{tpostings = map pivotPosting . tpostings $ t}
   pivotPosting p
     | Just (_ , value) <- tagTuple = p{paccount = value, porigin = Just $ originalPosting p}
-    | _                <- tagTuple = p
+    | _                <- tagTuple = p{paccount = T.pack ""}
    where tagTuple = find ((tag ==) . fst) . postingAllImplicitTags $ p
 
 -- | Apply the anonymisation transformation on a journal, if option is present

tests/misc/pivot.test

@@ -9,7 +9,7 @@ hledger -f- --pivot TAG print
     ; TAG: value
 >>>
 2016/02/16 Test Transaction
-    Account1         2 EUR
+                  2 EUR
     value        -2 EUR
     ; TAG: value
 
@@ -23,8 +23,8 @@ hledger -f- --pivot member bal --no-total
     income:member fees                                  -2 EUR
       ; member: John Doe
 >>>
+               2 EUR  
               -2 EUR  John Doe
-               2 EUR  assets:bank account
 >>>=0
 
 # check with another example
@@ -35,8 +35,8 @@ hledger -f- --pivot budget bal --no-total
     income:donations                                  -2 EUR
       ; budget: Freifunk
 >>>
+               2 EUR  
               -2 EUR  Freifunk
-               2 EUR  assets:bank account
 >>>=0
 
 # query on account will find either effective name or original one