[JENKINS-61711] Fix validateExecutable to allow Git plugin global config...

[JENKINS-61711] Fix validateExecutable to allow Git plugin global config correctly report git executable in PATH (#4653)

* Fix git not in path for Git plugin global config
Git Client Plugin global configuration allows addition of git implementations using GitTool
The field Path was not able to identify git executable. The fix involves changing prefix with
File.seperator instead of File.pathSeperator. This allows Jenkins to find the git exec at the
already existing path.

* Tests added to validate JENKINS-61711 fix

* Improve hamcrest comparison in FormValidationTest
Co-Authored-By: NMark Waite <mark.earl.waite@gmail.com>

* Import matchers for `not`
Co-authored-by: NMark Waite <mark.earl.waite@gmail.com>

core/src/main/java/hudson/util/DOSToUnixPathHelper.java

@@ -65,7 +65,7 @@ class DOSToUnixPathHelper {
 
                     tokenizedPathBuilder.append(_dir.replace('\\', '/'));
 
-                    if (checkPrefix(_dir + File.pathSeparator + exe, helper))
+                    if (checkPrefix(_dir + File.separator + exe, helper))
                         return;
                 }
                 tokenizedPathBuilder.append('.');

test/src/test/java/hudson/util/FormValidationTest.java

+package hudson.util;
+
+import org.junit.Rule;
+import org.junit.Test;
+import org.jvnet.hudson.test.Issue;
+import org.jvnet.hudson.test.JenkinsRule;
+
+import static org.hamcrest.Matchers.*;
+import static org.hamcrest.MatcherAssert.assertThat;
+
+public class FormValidationTest {
+
+    @Rule
+    public JenkinsRule j = new JenkinsRule();
+
+    @Issue("JENKINS-61711")
+    @Test
+    public void testValidateExecutableWithFix() {
+        // Global Tool Configuration is able to find git executable in system environment at PATH.
+        FormValidation actual = FormValidation.validateExecutable("git");
+        assertThat(actual, is(FormValidation.ok()));
+    }
+
+    @Issue("JENKINS-61711")
+    @Test
+    public void testValidateExecutableWithoutFix() {
+        // Without JENKINS-61711 fix, Git installations under Global Tool Configuration is not able to find git
+        // executable at system PATH despite git exec existing at the path.
+        FormValidation actual = FormValidation.validateExecutable("git");
+        String failMessage = "There's no such executable git in PATH:";
+        assertThat(actual, not(is(FormValidation.error(failMessage))));
+    }
+}