Installation Guide: mention that git 2.5.0 (2015) is needed server-side

Summary:
Note that the git version 2.5.0 is surely already assumed
as required as implicit fact, so it's just better to clarify
this in the documentation to prevent weird situations and
unclear related exceptions.

Note that if you have seen this Phabricator/Phorge error
in your installation:

This path was a submodule at R123:5378198ea123asdlol

This is probably that kind of weird errors that will be
fixed just updating your git version to 2.5.0, so that
the git escape sequence ('--') is correctly parsed by git.

Example of command that will fail with outdated git versions:

git cat-file -t -- <hash>:<file>

If you do not want to update git, but you want to fix,
you can convert that command to this one, manually patching
your installation:

git cat-file -t <hash>:<file>

Related material:

- https://we.phorge.it/T15179
- https://unix.stackexchange.com/a/740621/85666
- https://github.com/git/git/blob/6f9504c48ea59951a2aa3ee17e7c7fc36285e225/Documentation/RelNotes/2.5.0.txt

Wontfix T15179

Test Plan:
- open Diviner page "Installation Guide"
- see that the git version is not explicit - yup!

Reviewers: O1 Blessed Committers, Cigaryno, avivey

Reviewed By: O1 Blessed Committers, Cigaryno, avivey

Subscribers: speck, tobiaswiese, Matthew, Cigaryno

Maniphest Tasks: T15179

Differential Revision: https://we.phorge.it/D25081