Delete the three-space branch from the ps parser

`ps -o pid,command` has exactly one separator, so everything after the first
space is the command. The branch looking for a run of three spaces is left over
from a format that also carried TIME, and it was wrong in two ways at once.

It sliced timeAndCommandString by firstSpace - the PID's digit count, which is
not an index into that string at all - and then applied the index it found to
the unsliced span. So a command containing three consecutive spaces was
truncated to whatever followed them, and a seven digit PID with a short command
indexed past the end and threw ArgumentOutOfRangeException.

That throw comes out of ProcessCleanup's static constructor, so it is not one
bad line skipped: it is every launch and every kill in the process, permanently.

Two tests, both failing before this: a command with a run of spaces in it, and a
long PID with a short command.

Author: SimonCropp

src/DiffEngine.Tests/LinuxOsxProcessTests.cs

@@ -39,4 +39,34 @@ public async Task TryParse_singleDigit()
         await Assert.That(processCommand.Process).IsEqualTo(309);
         await Assert.That(processCommand.Command).IsEqualTo("System/Library/coreauthd -foo");
     }
+
+    /// <summary>
+    /// A command with a run of three spaces in it. The removed branch went looking for exactly
+    /// that and truncated the command to whatever followed it.
+    /// </summary>
+    [Test]
+    public async Task TryParse_commandContainingRunsOfSpaces()
+    {
+        var parse = LinuxOsxProcess.TryParse("123 /usr/bin/tool   file.txt", out var command);
+        await Assert.That(parse).IsTrue();
+        var processCommand = command!.Value;
+        await Assert.That(processCommand.Process).IsEqualTo(123);
+        await Assert.That(processCommand.Command).IsEqualTo("/usr/bin/tool   file.txt");
+    }
+
+    /// <summary>
+    /// A PID with more digits than the command has characters. The removed branch sliced by the
+    /// PID's digit count, which is not an index into this string at all, so this threw
+    /// ArgumentOutOfRangeException - and did so out of ProcessCleanup's static constructor, which
+    /// makes it permanent for the process.
+    /// </summary>
+    [Test]
+    public async Task TryParse_longPidShortCommand()
+    {
+        var parse = LinuxOsxProcess.TryParse("1234567 /x   y", out var command);
+        await Assert.That(parse).IsTrue();
+        var processCommand = command!.Value;
+        await Assert.That(processCommand.Process).IsEqualTo(1234567);
+        await Assert.That(processCommand.Command).IsEqualTo("/x   y");
+    }
 }

src/DiffEngine/Process/LinuxOsxProcess.cs

@@ -66,20 +66,15 @@ public static bool TryParse(string line, out ProcessCommand? processCommand)
             var pidString = trim[..firstSpace];
             var pid = int.Parse(pidString.ToString());
 
-            var timeAndCommandString = trim[(firstSpace + 1)..];
-            var multiSpaceIndex = 0;
-            CharSpan command;
-
-            var spaces = new CharSpan([' ',' ',' ']);
-            if (timeAndCommandString.IndexOf(spaces, StringComparison.InvariantCulture) > 0)
-            {
-                multiSpaceIndex = timeAndCommandString[firstSpace..].IndexOf(spaces, StringComparison.InvariantCulture);
-                command = timeAndCommandString[(multiSpaceIndex + 1)..].Trim();
-            }
-            else
-            {
-                command = timeAndCommandString[multiSpaceIndex..].Trim();
-            }
+            // `ps -o pid,command` has exactly one separator, so everything after the first space
+            // is the command. There used to be a second branch here looking for a run of three
+            // spaces, left over from a format that also carried TIME, and it was wrong twice over:
+            // it sliced by firstSpace, which is the PID's digit count and means nothing in this
+            // string, and then applied the index it found to the unsliced span. So a command
+            // containing three spaces was truncated, and a seven digit PID with a short command
+            // threw ArgumentOutOfRangeException - out of ProcessCleanup's static constructor,
+            // which makes it permanent for the process
+            var command = trim[(firstSpace + 1)..].Trim();
 
             processCommand = new(command.ToString(), in pid);
             return true;