Merge branch 'hv/mingw-help-is-executable'

"git help" enumerates executable files in $PATH; the implementation
of "is this file executable?" on Windows has been optimized.

* hv/mingw-help-is-executable:
  help: improve is_executable() on Windows

help.c

@@ -105,7 +105,22 @@ static int is_executable(const char *name)
 		return 0;
 
 #if defined(GIT_WINDOWS_NATIVE)
-{	/* cannot trust the executable bit, peek into the file instead */
+	/*
+	 * On Windows there is no executable bit. The file extension
+	 * indicates whether it can be run as an executable, and Git
+	 * has special-handling to detect scripts and launch them
+	 * through the indicated script interpreter. We test for the
+	 * file extension first because virus scanners may make
+	 * it quite expensive to open many files.
+	 */
+	if (ends_with(name, ".exe"))
+		return S_IXUSR;
+
+{
+	/*
+	 * Now that we know it does not have an executable extension,
+	 * peek into the file instead.
+	 */
 	char buf[3] = { 0 };
 	int n;
 	int fd = open(name, O_RDONLY);
@@ -113,8 +128,8 @@ static int is_executable(const char *name)
 	if (fd >= 0) {
 		n = read(fd, buf, 2);
 		if (n == 2)
-			/* DOS executables start with "MZ" */
-			if (!strcmp(buf, "#!") || !strcmp(buf, "MZ"))
+			/* look for a she-bang */
+			if (!strcmp(buf, "#!"))
 				st.st_mode |= S_IXUSR;
 		close(fd);
 	}