[api-merge] Normalize paths for comparisons (#1278)

The [macOS+xbuild build][0] is [broken][1]:

	obj/Debug/android-10/mcw/Android.Content.ContentProvider.cs(282,84): error CS0234: The type or namespace name 'Collections' does not exist in the namespace 'Android.System' (are you missing an assembly reference?)
	...and 1271 other similar errors...

[0]: https://jenkins.mono-project.com/view/Xamarin.Android/job/xamarin-android/855/
[1]: https://jenkins.mono-project.com/view/Xamarin.Android/job/xamarin-android/855/consoleText

The apparent cause is 7d705bf, which updated the build process to
generate `api.xml` files from `android-*.jar` at build time, instead
of requiring pre-generated paths within `src/Mono.Android/Profiles`,
as the `api-merge.exe` invocation was altered.

The *actual* cause is a latent bug in `api-merge.exe`: when using
`api-merge FILES --last-description=FILE`, `FILE` is incorrectly
compared with the list of `FILES`. When building with `xbuild`,
the `@(_AndroidProfile)` item group contains *full path names*, while
the `--last-description` option is a relative path:

	Executing: mono ../../bin/BuildDebug/api-merge.exe -o "obj/Debug/android-10/mcw/api.xml" \
	  -s '../../bin/BuildDebug/api/api-*.xml.in' \
	  …/xamarin-android/bin/BuildDebug/api/api-10.xml.in \
	  ...
	  --last-description=../../bin/BuildDebug/api/api-10.xml.in

Because `api-merge` didn't normalize the path names, it attempted to
compare `…/xamarin-android/bin/BuildDebug/api/api-10.xml.in` to
`../../bin/BuildDebug/api/api-10.xml.in`, which *always failed*.
Consequently, the resulting `obj/Debug/android-10/mcw/api.xml` file
contained *all* API levels, including the `android.system` package
(introduced in API-21), while not containing the namespace renaming
logic to map `android.system` to `Android.Systems`.

The result is that e.g.
obj/Debug/android-10/mcw/Android.Content.ContentProvider.cs`
contained "innocuous" identifiers such as
`System.Collections.Generic.IList<Android.Content.ContentProviderOperation>`
which were being incorrectly resolved to
`Android.System.Collections.Generic.IList<Android.Content.ContentProviderOperation>`,
which doesn't exist.

Fix `api-merge` so that it compares *full paths* to
`api-merge --last-description=FILE`, allowing the `api-merge` command
to properly stop merging APIs once it's hit (in this case)
`api-10.xml.in`.

Author: grendello

build-tools/api-merge/api-merge.cs

@@ -32,7 +32,7 @@ public static int Main (string[] args)
 				  v => glob = v },
 				{ "last-description=",
 				  "Last {DESCRIPTION} to process. Any later descriptions are ignored.",
-				  v => lastDescription = v },
+				  v => lastDescription = NormalizePath (v) },
 				{ "version",
 				  "Output version information and exit.",
 				  v => show_version = v != null },
@@ -69,14 +69,22 @@ public static int Main (string[] args)
 			SortSources (sources, glob);
 			ApiDescription context = new ApiDescription (sources [0]);
 			for (int i = 1; i < sources.Count; i++) {
-				if (sources [i-1] == lastDescription)
+				if (NormalizePath (sources [i-1]) == lastDescription)
 					break;
 				context.Merge (sources [i]);
 			}
 			context.Save (dest);
 			return 0;
 		}
 
+		static string NormalizePath (string path)
+		{
+			if (String.IsNullOrEmpty (path))
+				return path;
+
+			return Path.GetFullPath (path);
+		}
+
 		static void SortSources (List<string> sources, string globPattern)
 		{
 			if (globPattern == null)