I have found some answer in UE4.18 release note. The git features is enable by default if you have git folder in your project directory. This feature is used to exclude most iterated file from unity build. This is suppos decrease the compile (If you build the engine often indeed this feature is usefull, but in most case not).
To disable this behaviour you could change settings in BuildConfiguration.xml:
<SourceFileWorkingSet>
<Provider>None</Provider>
<RepositoryPath></RepositoryPath>
<GitPath></GitPath>
</SourceFileWorkingSet>
*Added support for adaptive non-unity builds when working from a Git repository. The ISourceFileWorkingSet interface is now used to query files belonging to the working set, and has separate implementations for Perforce (PerforceSourceFileWorkingSet) and Git (GitSourceFileWorkingSet). The Git implementation is used if a .git directory is found in the directory containing the Engine folder, the directory containing the project file, or the parent directory of the project file, and spawns a “git status” process in the background to determine which files are untracked or staged. Several new settings are supported in BuildConfiguration.xml to allow modifying default behavior: Default git *
Source : Unreal Engine 4.18 Released! - Announcements - Unreal Engine Forums