INSTALL.md: add link label def for `git-{commit,rebase}-mode'

Commit e2f9c456 added reference links for `git-commit-mode' and
`git-rebase-mode', but forgot to add the link label definition.

Signed-off-by: Pieter Praet <pieter@praet.org>
Merge pull request #832 from praet/doc-git-modes-link

INSTALL.md: add link label def for `git-{commit,rebase}-mode'