Sourcelink seems to break without a remote git repo

[TheAngryByrd]

Sourcelink seems to break without a remote git repo. Should try to figure out if there's a way to disable it until a remote has been used.

Starting Target: DotnetPack (==> DotnetTest)
dotnet "pack" "/Users/username/Documents/GitHub/someproj/src/someproj/someproj.fsproj" --configuration Release --output /Users/username/Documents/GitHub/someproj/dist /p:PackageVersion=0.1.0 /p:PackageReleaseNotes="Initial release" /p:SourceLinkCreate=true
Microsoft (R) Build Engine version 15.5.179.9764 for .NET Core
Copyright (C) Microsoft Corporation. All rights reserved.

  Restoring packages for /Users/username/Documents/GitHub/someproj/src/someproj/someproj.fsproj...
  Restore completed in 370.9 ms for /Users/username/Documents/GitHub/someproj/src/someproj/someproj.fsproj.
  fatal: Not a git repository (or any of the parent directories): .git
/Users/username/.nuget/packages/sourcelink.create.commandline/2.7.3/build/SourceLink.Create.CommandLine.targets(21,5): error MSB3073: The command "git rev-parse --show-toplevel" exited with code 128. [/Users/username/Documents/GitHub/someproj/src/someproj/someproj.fsproj]
  fatal: Not a git repository (or any of the parent directories): .git
/Users/username/.nuget/packages/sourcelink.create.commandline/2.7.3/build/SourceLink.Create.CommandLine.targets(21,5): error MSB3073: The command "git rev-parse --show-toplevel" exited with code 128. [/Users/username/Documents/GitHub/someproj/src/someproj/someproj.fsproj]
  fatal: Not a git repository (or any of the parent directories): .git
/Users/username/.nuget/packages/sourcelink.create.commandline/2.7.3/build/SourceLink.Create.CommandLine.targets(21,5): error MSB3073: The command "git rev-parse --show-toplevel" exited with code 128. [/Users/username/Documents/GitHub/someproj/src/someproj/someproj.fsproj]

Or without remote

/Users/username/.nuget/packages/sourcelink.create.commandline/2.7.3/build/SourceLink.Create.CommandLine.targets(24,5): error MSB3073: The command "git config --get remote.origin.url" exited with code 1. [/Users/username/Documents/GitHub/someproj/src/someproj/someproj.fsproj]

[TheAngryByrd]

Additionally #49 introduced another git dependency. Should check for git for this as well.