Refactor git version detection for reuse

This way we can use it to detect feature support in the underlying
git, such as new options or commands that have been added in more
recent versions.

Signed-off-by: Shawn O. Pearce <sop@google.com>

git_command.py

@@ -68,6 +68,30 @@ class _GitCall(object):
     return fun
 git = _GitCall()
 
+_git_version = None
+
+def git_require(min_version, fail=False):
+  global _git_version
+
+  if _git_version is None:
+    ver_str = git.version()
+    if ver_str.startswith('git version '):
+      _git_version = tuple(
+        map(lambda x: int(x),
+          ver_str[len('git version '):].strip().split('.')[0:3]
+        ))
+    else:
+      print >>sys.stderr, 'fatal: "%s" unsupported' % ver_str
+      sys.exit(1)
+
+  if min_version <= _git_version:
+    return True
+  if fail:
+    need = '.'.join(map(lambda x: str(x), min_version))
+    print >>sys.stderr, 'fatal: git %s or later required' % need
+    sys.exit(1)
+  return False
+
 class GitCommand(object):
   def __init__(self,
                project,

subcmds/init.py

@@ -20,7 +20,7 @@ from color import Coloring
 from command import InteractiveCommand, MirrorSafeCommand
 from error import ManifestParseError
 from project import SyncBuffer
-from git_command import git, MIN_GIT_VERSION
+from git_command import git_require, MIN_GIT_VERSION
 
 class Init(InteractiveCommand, MirrorSafeCommand):
   common = True
@@ -85,19 +85,6 @@ to update the working directory files.
                  dest='no_repo_verify', action='store_true',
                  help='do not verify repo source code')
 
-  def _CheckGitVersion(self):
-    ver_str = git.version()
-    if not ver_str.startswith('git version '):
-      print >>sys.stderr, 'error: "%s" unsupported' % ver_str
-      sys.exit(1)
-
-    ver_str = ver_str[len('git version '):].strip()
-    ver_act = tuple(map(lambda x: int(x), ver_str.split('.')[0:3]))
-    if ver_act < MIN_GIT_VERSION:
-      need = '.'.join(map(lambda x: str(x), MIN_GIT_VERSION))
-      print >>sys.stderr, 'fatal: git %s or later required' % need
-      sys.exit(1)
-
   def _SyncManifest(self, opt):
     m = self.manifest.manifestProject
     is_new = not m.Exists
@@ -214,7 +201,7 @@ to update the working directory files.
       gc.SetString('color.ui', 'auto')
 
   def Execute(self, opt, args):
-    self._CheckGitVersion()
+    git_require(MIN_GIT_VERSION, fail=True)
     self._SyncManifest(opt)
     self._LinkManifest(opt.manifest_name)