Similar to b33a15b08 (push: add recurseSubmodules config option,
2015-11-17) and 027771fcb1 (submodule: allow erroneous values for the
fetchRecurseSubmodules option, 2015-08-17), we add submodule-config code
that is later used to parse whether we are interested in updating
submodules.

We need the `die_on_error` parameter to be able to call this parsing
function for the config file as well, which if incorrect lets Git die.

As we're just touching the header file, also mark all functions extern.

Signed-off-by: Stefan Beller <redacted>
---
 submodule-config.c | 22 ++++++++++++++++++++++
 submodule-config.h | 17 +++++++++--------
 2 files changed, 31 insertions(+), 8 deletions(-)

diff --git a/submodule-config.c b/submodule-config.c
index 93453909cf..93f01c4378 100644
--- a/submodule-config.c
+++ b/submodule-config.c

@@ -234,6 +234,28 @@ int parse_fetch_recurse_submodules_arg(const char *opt, const char *arg)
        return parse_fetch_recurse(opt, arg, 1);
 }

+static int parse_update_recurse(const char *opt, const char *arg,
+                               int die_on_error)
+{
+       switch (git_config_maybe_bool(opt, arg)) {
+       case 1:
+               return RECURSE_SUBMODULES_ON;
+       case 0:
+               return RECURSE_SUBMODULES_OFF;
+       default:
+               if (!strcmp(arg, "checkout"))
+                       return RECURSE_SUBMODULES_ON;
+               if (die_on_error)
+                       die("bad %s argument: %s", opt, arg);
+               return RECURSE_SUBMODULES_ERROR;
+       }
+}