Improve update-assemblyinfo.ps1 x-plat support.

BuildTools/update-assemblyinfo.ps1

@@ -14,17 +14,24 @@ function Test-Dir([string]$name) {
 }
 
 function Find-Git() {
-	if ($env:PATH.Contains("git\cmd")) {
+	try {
-		return $true;
+		$executable = (get-command git).Path;
+		return $executable -ne $null;
+	} catch {
+		#git not found in path, continue;
 	}
+	#we're on Windows
+	if ($env:PROGRAMFILES -ne $null) {
 		#hack for x86 powershell used by default (yuck!)
 		if (${env:PROGRAMFILES(X86)} -eq ${env:PROGRAMFILES}) {
 			$env:PROGRAMFILES = $env:PROGRAMFILES.Substring(0, $env:PROGRAMFILES.Length - 6);
 		}
+		#try to add git to path
 		if ([System.IO.Directory]::Exists("$env:PROGRAMFILES\git\cmd\")) {
 			$env:PATH = "$env:PATH;$env:PROGRAMFILES\git\cmd\";
 			return $true;
 		}
+	}
 	return $false;
 }