* guix/scripts/pull.scm (%snapshot-url, with-environment-variable)
(with-PATH): Remove.
(ensure-guile-git!): New procedure.
(%repository-url): New variable.
(%default-options): Add 'repository-url' and 'ref'.
(show-help, %options): Add '--commit' and '--url'.
(temporary-directory, first-directory, interned-then-deleted)
(unpack): Remove.
(build-from-source): Rename 'tarball' to 'source'.  Remove call to
'unpack'.
(build-and-install): Rename 'tarball' to 'source'.
(honor-lets-encrypt-certificates!, report-git-error): New procedures.
(with-git-error-handling): New macro.
(guix-pull)[fetch-tarball]: Remove.
Wrap body in 'with-git-error-handling'.  Rewrite to use
'latest-repository-commit'.
* build-aux/build-self.scm (build): Print an error message and exit when
GUILE-GIT is #f.
* doc/guix.texi (Invoking guix pull): Mention Git.  Document '--commit'
and '--branch'.