[build] use relative paths for <GitBlame/> (#2594)

Downstream in monodroid, I'm getting a build failure such as:

	/usr/bin/git blame "/full/path/to/external/xamarin-android/Configuration.props"
	XAVersionInfo.targets(53,5): error MSB6006: "git" exited with code 128.
	XAVersionInfo.targets(61,5): error MSB4044: The "GitCommitsInRange" task was not given a value for the required parameter "StartCommit".

If I run the git command manually:

	fatal: not a git repository (or any of the parent directories): .git

Turns Out™ that some `git` versions are seemingly not happy when
given a full path name, i.e. this sequence of command can fail:

	$ cd /full/path/to/xamarin-android
	$ git blame `pwd`/Configuration.props

Appease these `git` versions and update the `<GitBlame>` invocation
to use a relative path, effectively doing:

	$ cd /full/path/to/xamarin-android
	$ git blame Configuration.props

Author: jonathanpeppers

build-tools/scripts/XAVersionInfo.targets

@@ -51,7 +51,7 @@
   <Target Name="GetXAVersionInfo"
       DependsOnTargets="_GetSubmodulesVersionInfo">
     <GitBlame
-        FileName="$(XamarinAndroidSourcePath)\Configuration.props"
+        FileName="Configuration.props"
         LineFilter="&lt;ProductVersion&gt;"
         WorkingDirectory="$(XamarinAndroidSourcePath)"
         ToolPath="$(GitToolPath)"

build-tools/xa-prep-tasks/Xamarin.Android.BuildTools.PrepTasks/GitBlame.cs

@@ -45,7 +45,7 @@ public override bool Execute ()
 
 		protected override string GenerateCommandLineCommands ()
 		{
-			return $"blame \"{Path.GetFullPath (FileName.ItemSpec)}\"";
+			return $"blame \"{FileName.ItemSpec}\"";
 		}
 
 		protected override void LogEventsFromTextOutput (string singleLine, MessageImportance messageImportance)