fix diff blame navigation

[gcb]

this does not "support" git options such as noprefix, mnemonicPrefix, etc. As in, they will not have the desired effect on diff view, like #1337 was trying to implement.

It just overrides them with the --default-prefix command line when getting the diffs, so blame navigation (e.g. pressing "b" in a diff) works.

fix #1336

[gcb]

About the failing tests... can either remove the existing places where diff prefixes are handled, or try to centralize them and cover all cases. e.g. https://github.com/jonas/tig/blob/tig-2.5.10/src/diff.c#L745 (there are others, searching for "prefix" on the log)

the places that have that code would still fail if .gitconfig have dstPrefix for example. ...and i couldn't find how to get whatever the heck is the prefix git will use, to avoid all that weird logic outside of git itself. In the end, i think forcing default-prefix is the safer way