Permit GIT_FAT_RSYNC_xyz env vars to override settings in .gitfat

[jmurty]

Add support for environment variables to override rsync settings from
the .gitfat file, for cases where you need to make ad-hoc config changes
or have differently named remote server names on different hosts, but
don't want to modify the common shared settings file.

[jedbrown]

I'm reluctant to create this interface. I'd rather create a git config section or a .git/info/fat for this sort of local modification. Would that serve your purpose?

[jmurty]

An alternative approach would be fine, especially one that fits better with standard git config handling. I'm not yet knowledgable enough about all git's config options to know exactly what should be done.

This isn't an urgent need. I anticipate needing something like it, but not immediately and it isn't hard to work around in the meantime.

Feel free to close this PR. I can take another swing at it later after some more research, if I find it necessary.

[duanem]

Interestingly I had the same problem. I was working with a bare repository so I could not incorporate a .gitfat file.

I added an option to git fat push. --rsync [filename]. The file is formatted like .gitfat but now can be taken from the command line.

I also like the git/info/fat. This was the only feature that I was not able to get working in a bare repository that made sense to use in a bare repository.