Allow supporting which branch to index in Context Provider

[gyxlucy]

Currently only the default branch is supported for indexing. Would be nice to allow users to choose which branch of code to index.

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Please reply with a 👍 if you want this feature.

[zwpaper]

I will work on this feature.

Here is the design draft for it.

Goals

1. Expose the indexed branches to user in UI/API, by default main for backward compatibility
2. Allow user to select more than one branches in UI/API for indexing
3. Files among branches would share the same embeddings/indexes if they were the same ones.
4. Only support starting to index by repo currently, branches will be handled one by one, will discuss starting indexing by each branch if necessary
5. Did NOT indexing the commit history from other branches, if this is needed, will discuss and implement in later.

Key changes

API

1. allow specifying the branches when adding repo for git, github, gitlab; main if omitted
   a.
   b.

2. allow user to update branches for existed repos
   a.

Indexing

1. add blob info for each file to indexing store, backfill to existing indexes when upgrading.
2. we already had commit info in index, will not add the branch info to it since it's volatile.