Use git ls-tree to autodetect build system

The cost of one git ls-tree call is roughly the same as one
git cat-file call. Therefore, when autodetecting the build system,
it is much faster to list the tree once and then search for the
required files than to call git cat-file for every possible one.

1 files changed, 3 insertions, 1 deletions

diff --git a/morphlib/buildsystem.py b/morphlib/buildsystem.py
index 8e213b75..86eac850 100644
--- a/morphlib/buildsystem.py
+++ b/morphlib/buildsystem.py
@@ -195,13 +195,15 @@ build_systems = [
 ]
 
 
-def detect_build_system(exists):
+def detect_build_system(file_list):
     '''Automatically detect the build system, if possible.
 
     If the build system cannot be detected automatically, return None.
     For ``exists`` see the ``BuildSystem.exists`` method.
 
     '''
+    def exists(filename):
+        return filename in file_list
 
     for bs in build_systems:
         if bs.used_by_project(exists):