build(tst-11): stop Windows builds stamping git stderr into InformationalVersion (NEXT-09)

MSBuildThisFileDirectory ends in a backslash, which escaped the closing quote
of the Exec command on Windows; git then failed and, because the target runs
with ContinueOnError + ConsoleToMSBuild (which mixes stderr into
ConsoleOutput), the failure text was stamped as the source revision — an
observed stamp read '0.1.2+fatal: cannot change to ...'. Append '.' to the
quoted path so the trailing separator can no longer escape the quote, and
gate SourceRevisionId on a short-SHA shape so no future git failure text can
become the revision either. Windows verification runs on windev with the
rest of this batch; macOS stamp confirmed unchanged (0.2.0+75c71ad).

src/Directory.Build.props

@@ -26,7 +26,10 @@
   <Target Name="StampSourceRevision"
           BeforeTargets="GetAssemblyVersion;GenerateAssemblyInfo"
           Condition="'$(SourceRevisionId)' == ''">
-    <Exec Command="git -C &quot;$(MSBuildThisFileDirectory)&quot; rev-parse --short HEAD"
+    <!-- The trailing "." is load-bearing: $(MSBuildThisFileDirectory) ends in a path
+         separator, and on Windows that trailing backslash escapes the closing quote,
+         mangling the command so git's stderr got stamped as the revision (NEXT-09). -->
+    <Exec Command="git -C &quot;$(MSBuildThisFileDirectory).&quot; rev-parse --short HEAD"
           ConsoleToMSBuild="true"
           StandardOutputImportance="Low"
           ContinueOnError="true"
@@ -34,7 +37,9 @@
       <Output TaskParameter="ConsoleOutput" PropertyName="_StampedGitSha" />
     </Exec>
     <PropertyGroup>
-      <SourceRevisionId Condition="'$(_StampedGitSha)' != ''">$(_StampedGitSha.Trim())</SourceRevisionId>
+      <!-- Accept only something that looks like a git short SHA; Exec's ConsoleOutput
+           mixes in stderr, so any git failure text must never become the revision. -->
+      <SourceRevisionId Condition="$([System.Text.RegularExpressions.Regex]::IsMatch('$(_StampedGitSha.Trim())', '^[0-9a-f]{7,40}$'))">$(_StampedGitSha.Trim())</SourceRevisionId>
     </PropertyGroup>
   </Target>