[Diff] The staged diff is lost when a file is written to disk

[StanAngeloff]

With the Staged item selected under Status, a diff of all staged changes is shown in the Diff panel. However, if a file is saved to disk, the diff is lost and I have to press Ctrl+R to restore it:

This happens on Linux with the latest version of git-cola running from master:

➜ python -V
Python 2.7.6

➜ git-cola --version
cola version 2.6

➜ git rev-parse HEAD
ac0dd100dbaf4c94cff2795f52c86746b13aeb6c

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Thank you for working on git-cola. It is the only Git GUI that I've grown to love and use everyday! ✅