Use environment variables when creating signatures by u-quark · Pull Request #6706 · libgit2/libgit2

When creating an action signature (e.g. for a commit author and
committer) read the following environment variables that can override
the configuration options:

 * `GIT_AUTHOR_NAME` is the human-readable name in the "author" field.
 * `GIT_AUTHOR_EMAIL` is the email for the "author" field.
 * `GIT_AUTHOR_DATE` is the timestamp used for the "author" field.
 * `GIT_COMMITTER_NAME` sets the human name for the "committer" field.
 * `GIT_COMMITTER_EMAIL` is the email address for the "committer" field.
 * `GIT_COMMITTER_DATE` is used for the timestamp in the "committer"
   field.
 * `EMAIL` is the fallback email address in case the user.email
   configuration value isn't set. If this isn't set, Git falls back to
   the system user and host names.

This is taken from the git documentation chapter "10.8 Environment
Variables":

https://git-scm.com/book/en/v2/Git-Internals-Environment-Variables

This PR adds support for reading these environment variables by adding
two new functions `git_signature_default_author` and
`git_signature_default_committer` and deprecates the
`git_signature_default` function.

Fixes: libgit2#3751

Prior work:
 * libgit2#4409
 * libgit2#5479
 * libgit2#6290

Also fix some comment formatting.

Making the various pieces that create commits automatically (eg, rebase)
start paying attention to the environment variables is a Big Change.
For now, this is a big change in defaults; we should treat it as
breaking. We don't move to this by default; we may add `from_env` or
`honor_env` type of API surface in the future.

People who are doing a commit expect a unified timestamp between
author and committer information when we're using the current timestamp.
Provide a single function that returns both author and committer
information so that they can have an identical timestamp when none is
specified in the environment.