Location in GHCi debugger prompt printed twice when default prompt is used

(Noticed in #8316)

Test.hs:

foo :: [Int]
foo = [ 1 .. 10 ]

Reproducer:

$ ghc-stage2 --interactive -ignore-dot-ghci Test.hs
GHCi, version 8.5.20180325: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Main             ( Test.hs, interpreted )
Ok, one module loaded.
*Main> :break foo
Breakpoint 0 activated at Test.hs:2:7-14
*Main> foo
Stopped in Main.foo, Test.hs:2:7-14
_result :: [Int] = _
[Test.hs:2:7-14] [Test.hs:2:7-14] *Main>

The [Test.hs:2:7-14] part is repeated. Setting the prompt fixes it:

$ ghc-stage2 --interactive Test.hs -ignore-dot-ghci
GHCi, version 8.5.20180325: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Main             ( Test.hs, interpreted )
Ok, one module loaded.
*Main> :set prompt "> "
> :break foo
Breakpoint 0 activated at Test.hs:2:7-14
> foo
Stopped in Main.foo, Test.hs:2:7-14
_result :: [Int] = _
[Test.hs:2:7-14] >

Trac metadata

Trac field	Value
Version	8.5
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	GHCi
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