encapsulate tidbits' .gitconfig in a separate file

[brandonzylstra]

Rather than inserting the configuration for Git inside the user's global configuration (~/.gitconfig), it seems preferable to include it by simply adding something like

[include]
    path = ~/.tidbits/gitconfig

(For reference, see: git/git@9b25a0b)

The main problem with overloading the user's ~/.gitconfig is that we can easily wind up with instances where there are multiple values for one configuration setting, which can prevent the git config command from functioning. For instance, suppose both the Tidbits section and the rest of a config file both have:

[example "test"]
  blah = yada

and the user runs a script that executes git config --global example.test.blah yackety, they will get an error:

warning: example.test.blah has multiple values
error: cannot overwrite multiple values with a single value
       Use a regexp, --add or --replace-all to change example.test.blah.

Inclusion of external config files should work in Git 1.7.10.2 and up. If we wanted to support earlier versions (and I don't think we care about them) we could put the Tidbits configurations in /etc/gitconfig (or just maintain the current behavior for ancient versions of Git).

[brandonzylstra]

aha.. just noticed that this appears to be a known issue. This is from the text that is inserted at the top of ~/.gitconfig:

##############################
# POTENTIAL CONFLICTS WITH GIT
##############################
# git-config commands like add/remove are not smart enough to add/remove settings
# from the proper place in your ~/gitconfig file when using shared configs. You may
# need to add/remove them manually. If you store your ~/.gitconfig file in a git repository
# itself then it will be extremely easy to see if unexpected changes occurred. For more
# information on how-to do that contact your local git ninja.