Always decode the first two bytes of a commit ID into ushort as big endian

This fixes a non-deterministic version computation that varied based on endianness of the processor. Most computers use little-endian, which produced a decimal version that when converted back to hex with a typical calculator would have the first two commit ID bytes in swapped positions.
Now with this change, we "fix" the endianness. We choose big endian since it won't swap the first two bytes like little endian did.

But this presents another problem: all the generated versions now have a different value for the 4th integer component, and `nbgv get-commits` will no longer match a commit when it built using the little-endian version of NB.GV. I'll fix this in a subsequent commit so that this CLI tool will match on commits allowing the order to be swapped.

Fixes #637

Author: AArnott

src/NerdBank.GitVersioning.Tests/ManagedGit/GitObjectIdTests.cs

@@ -93,7 +93,7 @@ public void AsUInt16Test()
         {
             // The hash code is the int32 representation of the first 4 bytes
             var objectId = GitObjectId.ParseHex(this.shaAsHexAsciiByteArray);
-            Assert.Equal(0x914e, objectId.AsUInt16());
+            Assert.Equal(0x4e91, objectId.AsUInt16());
             Assert.Equal(0, GitObjectId.Empty.GetHashCode());
         }
 

src/NerdBank.GitVersioning.Tests/VersionOracleTests.cs

@@ -845,6 +845,26 @@ public void GitCommitIdShort()
         }
     }
 
+    [Fact]
+    public void GitCommidIdLeading16BitsDecodedWithBigEndian()
+    {
+        this.WriteVersionFile(new VersionOptions { Version = SemanticVersion.Parse("1.2"), GitCommitIdShortAutoMinimum = 4 });
+        this.InitializeSourceControl();
+        this.AddCommits(1);
+        var oracle = new VersionOracle(this.Context);
+
+        string leadingFourChars = this.Context.GitCommitId.Substring(0, 4);
+        ushort expectedNumber = FromHex(leadingFourChars);
+        ushort actualNumber = checked((ushort)oracle.Version.Revision);
+        this.Logger.WriteLine("First two characters from commit ID in hex is {0}", leadingFourChars);
+        this.Logger.WriteLine("First two characters, converted to a number is {0}", expectedNumber);
+        this.Logger.WriteLine("Generated 16-bit ushort from commit ID is {0}, whose hex representation is {1}", actualNumber, ToHex(actualNumber));
+        Assert.Equal(expectedNumber, actualNumber);
+
+        static string ToHex(ushort number) => number.ToString("X");
+        static ushort FromHex(string hex) => ushort.Parse(hex, System.Globalization.NumberStyles.HexNumber);
+    }
+
     [Fact(Skip = "Slow test")]
     public void GetVersionHeight_VeryLongHistory()
     {

src/NerdBank.GitVersioning/LibGit2/LibGit2GitExtensions.cs

@@ -3,8 +3,8 @@
 namespace Nerdbank.GitVersioning.LibGit2
 {
     using System;
+    using System.Buffers.Binary;
     using System.Collections.Generic;
-    using System.Globalization;
     using System.IO;
     using System.Linq;
     using System.Runtime.InteropServices;
@@ -106,7 +106,7 @@ public static int GetHeight(LibGit2Context context, Func<Commit, bool>? continue
         public static ushort GetTruncatedCommitIdAsUInt16(this Commit commit)
         {
             Requires.NotNull(commit, nameof(commit));
-            return BitConverter.ToUInt16(commit.Id.RawId, 0);
+            return BinaryPrimitives.ReadUInt16BigEndian(commit.Id.RawId);
         }
 
         /// <summary>
@@ -301,7 +301,7 @@ private static bool IsCommitIdMismatch(Version version, VersionOptions versionOp
         /// <returns><c>True</c> if the object's ID starts with <paramref name="leadingBytes"/> after applying the <paramref name="bitMask"/>.</returns>
         private static bool StartsWith(this ObjectId @object, ushort leadingBytes, ushort bitMask = 0xffff)
         {
-            ushort truncatedObjectId = BitConverter.ToUInt16(@object.RawId, 0);
+            ushort truncatedObjectId = BinaryPrimitives.ReadUInt16BigEndian(@object.RawId);
             return (truncatedObjectId & bitMask) == leadingBytes;
         }
 

src/NerdBank.GitVersioning/ManagedGit/GitObjectId.cs

@@ -167,7 +167,7 @@ public override bool Equals(object? obj)
         /// <returns>
         /// A <see cref="ushort"/> which represents the first two bytes of this <see cref="GitObjectId"/>.
         /// </returns>
-        public ushort AsUInt16() => BinaryPrimitives.ReadUInt16LittleEndian(this.Value.Slice(0, 2));
+        public ushort AsUInt16() => BinaryPrimitives.ReadUInt16BigEndian(this.Value.Slice(0, 2));
 
         /// <summary>
         /// Returns the SHA1 hash of this object.