The function is currently only being used for checking the result of
`git rev-parse --is-inside-work-tree`. Checking for the return code of
the command is enough.
* remove unneeded explination and authorship comments (as it's
noted in the README.md)
* s/git/command git/ (see issue #30)
* add is-true function, as it is required by a few other functions.
Eric Nielsen
Sebastian S
Matt Hamilton