Fix a few issues with git renames handling

[matiasb]

Supersedes #76 and fixes both issues described in #77.

[krobelus]

Looks good, thanks.

Looking at Git's diff_set_mnemonic_prefix, they sometimes set different prefixes instead of a/ and b/, for example in git status -vv. The full list should be 12abciow, though I don't know if there's a real use case that would benefit from that..

[matiasb]

Looking at Git's diff_set_mnemonic_prefix, they sometimes set different prefixes instead of a/ and b/, for example in git status -vv. The full list should be 12abciow, though I don't know if there's a real use case that would benefit from that..

That's good to know. Making the changes to support the different variations will require additional work in several places (there is code already assuming a/ and b/ prefixes). Adding an issue (#81) to track and eventually handle this