Include the git blob id of the dir-index bundle in the ETag

[mib-kd743naq]

While the content of raw files retrieved via the gateway should never
change, the look and feel of the directory index can and will change
between versions of go-ipfs.

Incorporate the hash of assets/bindata.go into the ETag when appropriate

License: MIT
Signed-off-by: Mib Kd743naq mib.kd743naq@gmail.com

[mib-kd743naq]

Forgot to add a visual of what this does.

Before:

~$ curl -I 127.0.0.1:8080/ipfs/QmS4ustL54uo8FzR9455qaxZwuMiUhyvMcX9Ba8nUH4uVv
HTTP/1.1 200 OK
...
Etag: "QmS4ustL54uo8FzR9455qaxZwuMiUhyvMcX9Ba8nUH4uVv"

After:

~$ curl -I 127.0.0.1:8080/ipfs/QmS4ustL54uo8FzR9455qaxZwuMiUhyvMcX9Ba8nUH4uVv
HTTP/1.1 200 OK
...
Etag: "DirIndex-ad00a51de6a0a134329b35f56612d3adfb3972d9_CID-QmS4ustL54uo8FzR9455qaxZwuMiUhyvMcX9Ba8nUH4uVv"

[Stebalien]

I'd like to avoid relying on someone running the makefile. IIRC, even our binary distribution system manually builds with go build.

What if we either:

1. Compute this on init (not that expensive).
2. Add a go generate rule to assets/assets.go (preferred).

[mib-kd743naq]

even our binary distribution system manually builds with go build.

Whoa! I didn't know this :) The README implies make is the one and only way. I like option 2 as well, will close the PR for now, will reopen when I have a chance to redo.

[Stebalien]

Well, make is the correct way but I think this is a case of "we know better so we're going to ignore our own best practice".