Appveyor: add prepareDeploy.ps1 and gitDescribe.ps1

which is needed to use GitHub API to correctly tag automatic
builds, because git-describe doesn't work on shallow clones

.appveyor/gitDescribe.ps1

@@ -0,0 +1,54 @@
+$currentCommitSha = $Env:CurrentCommitSha
+$githubRepoApiUri = $Env:githubRepoApiUri #"https://api.github.com/repos/:user/:repo/"
+$Env:VersionTag = ""
+
+# Inline-if helper function
+Function IIf($If, $Right, $Wrong) {If ($If) {$Right} Else {$Wrong}}
+
+try{
+	$result = Invoke-RestMethod -Uri "$($githubRepoApiUri)git/refs/tags" -Method 'GET'
+}
+catch [System.Net.WebException] 
+{	
+	# Error getting tags
+    $statusCode = [int]$_.Exception.Response.StatusCode
+    $html = $_.Exception.Response.StatusDescription
+	Write-Host "Error getting tags ($html)"
+	Return ;
+}
+
+# Get tag info
+try{
+	$tagsInfo = ($result | % { "$($githubRepoApiUri)git/tags/$($_.object.sha)" ;} | % { Invoke-RestMethod -Method 'GET' -Uri $($_)})
+}
+catch [System.Net.WebException] 
+{	
+	# Error getting tags
+    $statusCode = [int]$_.Exception.Response.StatusCode
+    $html = $_.Exception.Response.StatusDescription
+	Write-Host "Error getting tags ($html)"
+	Return ;
+}
+
+# Compare taged commits with current commit
+try{
+	$commitTagComp = $tagsInfo | select sha, tag, tagger, @{n='dist';e={[int](Invoke-RestMethod -Method 'GET' -Uri "$($githubRepoApiUri)compare/$($_.object.sha)...$currentCommitSha" | % { (IIf ($_.status -ne "diverged") ($_.ahead_by) (-1))})}}
+}
+catch [System.Net.WebException] 
+{	
+	# Error comparing tags
+    $statusCode = [int]$_.Exception.Response.StatusCode
+    $html = $_.Exception.Response.StatusDescription
+	Write-Host "Error comparing tags ($html)"
+	Return ;
+}
+
+$latestTag = $commitTagComp | where {$_.dist -ge 0} | sort-object -property @{Expression="dist";Descending=$false}, @{Expression="tag";Descending=$false} | Select-Object -first 1
+
+if(!$latestTag){
+	Write-Host "Commit not describable"	
+	Return ;
+}
+
+$Env:VersionTag = "$($latestTag.tag)-$($latestTag.dist)-g$($currentCommitSha.Substring(0,7))"
+Return $Env:VersionTag