Make Query.query('exist') lookup faster by adding a filename cache

Add a cache for `exists` queries. Currently, `exists` calls `git
ls-tree` and parses the result to check if a file exists. A single
call takes around 20-30 ms.

It only gets used by Makefile filters. Large Makefiles cause a filter to
make hundreds of these calls, causing filter processing to take
seconds.

Statistics on 20 HTTP requests on /linux/v6.11.6/source/MAINTAINERS:

                 without:     with:
    avg            1160        843
    median          951        790
    75th perc      1289        861
    95th perc      2452       1078
    max            2874       1749

The cache is stored inside Query, of which there is one instance per
request. We do not risk cache invalidation issues.

About memory usage: on Linux v6.9.4, the cache is 12MB.

elixir/query.py

@@ -61,6 +61,7 @@ def __init__(self, data_dir, repo_dir):
         self.data_dir = data_dir
         self.dts_comp_support = int(self.script('dts-comp'))
         self.db = data.DB(data_dir, readonly=True, dtscomp=self.dts_comp_support)
+        self.file_cache = {}
 
     def script(self, *args):
         return script(*args, env=self.getEnv())
@@ -136,22 +137,22 @@ def query(self, cmd, *args):
             return decode(self.script('get-type', version, path)).strip()
 
         elif cmd == 'exist':
-
-            # Returns True if the requested file exists, otherwise returns False
-
             version = args[0]
             path = args[1]
 
-            dirname, filename = os.path.split(path)
-
-            entries = decode(self.script('get-dir', version, dirname)).split("\n")[:-1]
-            for entry in entries:
-                fname = entry.split(" ")[1]
+            if version not in self.file_cache:
+                version_cache = set()
+                last_dir = None
+                for _, path in self.db.vers.get(version).iter():
+                    dirname, filename = os.path.split(path)
+                    if dirname != last_dir:
+                        last_dir = dirname
+                        version_cache.add(dirname)
+                    version_cache.add(path)
 
-                if fname == filename:
-                    return True
+                self.file_cache[version] = version_cache
 
-            return False
+            return path.strip('/') in self.file_cache[version]
 
         elif cmd == 'dir':