format: idxfile, support for >2Gb packfiles

[mcuadros]

Fixes #502

-rw-r--r-- 1 mcuadros users 1.4K Jul 26 20:12 pack-63303cff443f77256986ca08885584fb90e87a7c.idx
-rw------- 1 mcuadros users 5.6G Jul 26 20:11 pack-63303cff443f77256986ca08885584fb90e87a7c.pack

> git verify-pack -v .git/objects/pack/pack-63303cff443f77256986ca08885584fb90e87a7c.idx
303953e5aa461c203a324821bc1717f9b4fff895 commit 183 130 12
35858be9c6f5914cbe6768489c41eb6809a2bceb tree   376 312 142
5296768e3d9f661387ccbff18c4dea6c997fd78c blob   1661769914 1601322695 454
03fc8d58d44267274edef4585eaeeb445879d33f blob   1047302104 1045673692 1601323149
8f3ceb4ea4cb9e4a0f751795eb41c9a4f07be772 blob   807073668 805389077 2646996841
e0d1d625010087f79c9e01ad9d8f95e1628dda02 blob   266246426 254661864 3452385918
90eba326cdc4d1d61c5ad25224ccbf08731dd041 blob   1664417792 1616175862 3707047782
bab53055add7bc35882758a922c54a874d6b1272 blob   580376607 570849611 5323223644
1b8995f51987d8a449ca5ea4356595102dc2fbd4 blob   30310348 30205976 5894073255
non delta: 9 objects

[smola]

s/Big ndian/Big endian/

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@c256136). Click here to learn what that means.
The diff coverage is 50%.

@@            Coverage Diff            @@
##             master     #512   +/-   ##
=========================================
  Coverage          ?   77.91%           
=========================================
  Files             ?      128           
  Lines             ?     9497           
  Branches          ?        0           
=========================================
  Hits              ?     7400           
  Misses            ?     1312           
  Partials          ?      785

Impacted Files	Coverage Δ
plumbing/format/idxfile/idxfile.go	90.47% <ø> (ø)
utils/binary/read.go	62.9% <0%> (ø)
utils/binary/write.go	80% <0%> (ø)
plumbing/format/idxfile/encoder.go	75.67% <70%> (ø)
plumbing/format/idxfile/decoder.go	63.15% <71.42%> (ø)

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[ajnavarro]

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