Replace deprecated git repo-config

[adridg]

Summary

repo-config is no longer supported by any upstream-supported
git version. Instead, use git config (which is implicitly
git config --file, corresponding to the earlier config-in-repo).

Every time you start qgit, or refresh a repo, you get a popup
saying that repo-config is not recognized. This makes qgit
annoying to use at best. Unless you have a really old git
installed, the intent of the config option is broken.

Test Plan

Builds for me (amd64, 10.3). Runs, too, without the annoying popup.
Since I don't have any repo's that actually use commitEncoding,
I don't have much to user-test with -- but given that after the
popup it was falling back to utf-8 anyway, this can't be a
regression.

[adridg]

Maintainer of the port is listed as itetcu@, but that doesn't work in Phab -- adding some KDE peeps instead.

[tcberner]

There is also a commit for this in the 'official' github repo
https://github.com/tibirna/qgit/commit/6a4f1086114478e5f0be9929da92aad42123d988

As your patch however contains a descriptive comment, and it's not quite clear how 'official' the so called 'official' repository on github actually is, I think it's ok to just use your patch :)

[rakuco]

lgtm. @tcberner: feel free to land this and close any PR that might exist about this (don't forget to MFH the change).