Update installation.md

working fix for issue #7794
https://www.kernel.org/pub/software/scm/git/docs/git-config.html

>core.precomposeunicode 
This option is only used by Mac OS implementation of git. When core.precomposeunicode=true, git reverts the unicode decomposition of filenames done by Mac OS. This is useful when sharing a repository between Mac OS and Linux or Windows. (Git for Windows 1.7.10 or higher is needed, or git under cygwin 1.7). When false, file names are handled fully transparent by git, which is backward compatible with older versions of git.

@sjbog are you running GitLab on OS X?