config.c: add git_env_ulong() to parse environment variable

The new function parses an integeral value that fits in unsigned
long in human readable form, i.e. possibly with unit suffix, e.g.
10k = 10240, etc., from an environment variable.  Parsing of
GIT_MMAP_LIMIT and GIT_ALLOC_LIMIT will use it in later patches.

Signed-off-by: Steffen Prohaska <prohaska@zib.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

config.c

@@ -1116,12 +1116,28 @@ const char *git_etc_gitconfig(void)
 	return system_wide;
 }
 
+/*
+ * Parse environment variable 'k' as a boolean (in various
+ * possible spellings); if missing, use the default value 'def'.
+ */
 int git_env_bool(const char *k, int def)
 {
 	const char *v = getenv(k);
 	return v ? git_config_bool(k, v) : def;
 }
 
+/*
+ * Parse environment variable 'k' as ulong with possibly a unit
+ * suffix; if missing, use the default value 'val'.
+ */
+unsigned long git_env_ulong(const char *k, unsigned long val)
+{
+	const char *v = getenv(k);
+	if (v && !git_parse_ulong(v, &val))
+		die("failed to parse %s", k);
+	return val;
+}
+
 int git_config_system(void)
 {
 	return !git_env_bool("GIT_CONFIG_NOSYSTEM", 0);