Share git version parsing code with wrapper module

'repo' and 'git_command.py' had their own git version parsing code.
This change shares that code between the modules.  DRY is good.

Change-Id: Ic896d2dc08353644bd4ced57e15a91284d97d54a

repo

@@ -278,6 +278,20 @@ def _Init(args):
     raise
 
 
+def ParseGitVersion(ver_str):
+  if not ver_str.startswith('git version '):
+    return None
+
+  num_ver_str = ver_str[len('git version '):].strip().split('-')[0]
+  to_tuple = []
+  for num_str in num_ver_str.split('.')[:3]:
+    if num_str.isdigit():
+      to_tuple.append(int(num_str))
+    else:
+      to_tuple.append(0)
+  return tuple(to_tuple)
+
+
 def _CheckGitVersion():
   cmd = [GIT, '--version']
   try:
@@ -295,12 +309,11 @@ def _CheckGitVersion():
   proc.stdout.close()
   proc.wait()
 
-  if not ver_str.startswith('git version '):
+  ver_act = ParseGitVersion(ver_str)
+  if ver_act is None:
     _print('error: "%s" unsupported' % ver_str, file=sys.stderr)
     raise CloneFailure()
 
-  ver_str = ver_str[len('git version '):].strip()
-  ver_act = tuple(map(int, ver_str.split('.')[0:3]))
   if ver_act < MIN_GIT_VERSION:
     need = '.'.join(map(str, MIN_GIT_VERSION))
     _print('fatal: git %s or later required' % need, file=sys.stderr)