Add tagopt parsing support to RemoteConfig
C Git permits the user to specify how tags should be handled during
fetch by setting remote.*.tagopt in their .git/config. Our TagOpt
enum represents this same range of values, along with the translation
to/from the C Git preferred setting, which for historical reasons
is a GNU style --long-option rather than a simple string.
We don't actually honor this setting yet in our fetch implementation,
but we can read and write it now.
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>