fix: shorten GetPipeName() output

[Thundernerd]

Redo of #2579 with correct target branch.

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This is a hotfix to combat the hard crash on unix machines which was introduced by e0e2ab6

On Unix machines the NamedPipeServerStream is created with a Path.GetTempPath() and "CoreFXPipe_" prefix, causing the recent change to go over the maximum length of the pipename of 104 characters: dotnet Unix PipeStream sourcecode.

Note
In the case of a long username this can still break as we now add an extra 16 characters from the hash to this pipe name.

Closes #2576

[Thundernerd]

Like @JC-Chung suggested, reduced the hash length to 8 characters.
Also renamed from SGit to SourceGit again for clarity, which adds 4 characters.
In total now it has been reduced by 14 characters

[Mingun]

In the case of a long username this can still break as we now add an extra 16 characters from the hash to this pipe name.

That problem can be totally eliminated if include the user name in hash. What the point to keep it readable in this internal name?