Add colors support in tig grep

[ael-code]

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Currently the grep view doesn't highlight matched string. This is because by default the --no-color option is used for the underlying git grep command: https://github.com/jonas/tig/blob/b2694de4055f9ecb249f82dbb6ed8275fc2e1984/src/grep.c#L90

If I override the command line parameters for git grep with: tig grep --color <match> the ANSI escape characters are not interpreted by tig and are printed as normal string.

Is there any way to make tig grep interpret ANSI escape chars?