Preparations for exceptions on git error (part 3 of 4)

[mstv]

Some of the preparations for #9056 contributed by @gerhardol and me

Proposed changes

• Fixup GitModule.UpdateIndex: output the newline after the file name
  in order to avoid a git error message on the first file name being empty
• AheadBehindDataProvider: Return null on non-zero git exit code

Screenshots

not affected

Test methodology

• NUnit tests
• manual

Test environment(s)

• Git Extensions 33.33.33
• Build 35632c7
• Git 2.31.1.windows.1
• Microsoft Windows NT 10.0.19042.0
• .NET 5.0.7
• DPI 96dpi (no scaling)

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✒️ I contribute this code under The Developer Certificate of Origin.