bug: fail to find filename if not showing prefixes

[gcb]

1. git config diff.noprefix true
2. open tig blame
3. select a diff section and press b

what i expect:
To be magically taken to the previous review on the diff.

what happens:

Failed to read the file name

(very minor bug. mostly to document to future me when i see that error again)

[koutcher]

Fixed by 2a8ef0f.