Make use of stat.ctime configurable

A new configuration variable 'core.trustctime' is introduced to
allow ignoring st_ctime information when checking if paths
in the working tree has changed, because there are situations where
it produces too much false positives.  Like when file system crawlers
keep changing it when scanning and using the ctime for marking scanned
files.

The default is to notice ctime changes.

Signed-off-by: Alex Riesen <raa.lkml@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

environment.c

@@ -13,6 +13,7 @@ char git_default_email[MAX_GITNAME];
 char git_default_name[MAX_GITNAME];
 int user_ident_explicitly_given;
 int trust_executable_bit = 1;
+int trust_ctime = 1;
 int has_symlinks = 1;
 int ignore_case;
 int assume_unchanged;