Add git-crypt support

[robpou]

This commit tries to add support for git-crypt. While I'm aware that both git-secret and sops are already supported, our team (and I'm sure many others) rely on git-crypt and since the additional support is quite trivial, I'm asking for including it.

We are in progress of moving our processes to flux, and as we saw that you support git-secret our first thought was simply "ok let's change our repo to it then". But the more we had to work with it the less we liked it. The main reasons are:

• every commit (git secret reveal && editing something && git secret hide) results by default in a re-encryption of all of the secret files (in our case more than 100 files) which pollutes history
• git-secret relies on .gitignore entries, which actually hide the uncommited changes e.g. from git status
• git diff is useless with git-secret (the workaround painful e.g. https://github.com/msilvestre/git-secret-diff)

First #1676 asked for crypto support, but that ended up implementing git-secret in #2159. This commit would also solve conversations like #2904

Please tell me if I can improve something and I will.

[robpou]

I saw the CI error and will fix the issue with the missing function.
Should I close this and create a new MR or update this one?

[cedi]

this line should be deleted, as it causes a syntax error:
two opening if's but only one end.
This line is made obsolete by the next line :)