git_config: add GetInt helper

Change-Id: Ic034ae2fd962299d1b352e597b391b6582ecf44b
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/256052
Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: Jonathan Nieder <jrn@google.com>

tests/fixtures/test.gitconfig

@@ -1,3 +1,13 @@
 [section]
 	empty
 	nonempty = true
+	boolinvalid = oops
+	booltrue = true
+	boolfalse = false
+	intinvalid = oops
+	inthex = 0x10
+	inthexk = 0x10k
+	int = 10
+	intk = 10k
+	intm = 10m
+	intg = 10g