Update installation.md

Updated the version of Git used in the section for compiling Git from source.

Also, changed the verbiage used in the section "Initialize Database and Activate Advanced Features". It previously said to type 'yes' to install the database.
Include a note for people that think the headers are wrong.
Version to 6.6.0.pre

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
 Merge branch 'database-confusion' into 'master'

Database Confusion

To prevent https://twitter.com/TechIsCool/status/426133762032676865
Merge pull request #5907 from boomshadow/patch-1

Update installation.md with new git version