Better fix for isalpha in drive letter detection

Missed a place that used this and missed git__isalpha

src/path.c

@@ -17,6 +17,10 @@
 #include <stdio.h>
 #include <ctype.h>
 
+#ifdef GIT_WIN32
+#define LOOKS_LIKE_DRIVE_PREFIX(S) (git__isalpha((S)[0]) && (S)[1] == ':')
+#endif
+
 /*
  * Based on the Android implementation, BSD licensed.
  * Check http://android.git.kernel.org/
@@ -105,7 +109,7 @@ int git_path_dirname_r(git_buf *buffer, const char *path)
 	/* Mimic unix behavior where '/.git' returns '/': 'C:/.git' will return
 		'C:/' here */
 
-	if (len == 2 && isalpha(path[0]) && path[1] == ':') {
+	if (len == 2 && LOOKS_LIKE_DRIVE_PREFIX(path)) {
 		len = 3;
 		goto Exit;
 	}
@@ -170,8 +174,7 @@ int git_path_root(const char *path)
 
 #ifdef GIT_WIN32
 	/* Does the root of the path look like a windows drive ? */
-	if (((path[0] >= 'a' && path[0] <= 'z') ||
-		 (path[0] >= 'A' && path[0] <= 'Z')) && path[1] == ':')
+	if (LOOKS_LIKE_DRIVE_PREFIX(path))
 		offset += 2;
 
 	/* Are we dealing with a windows network path? */