Fix indent calculation with tabs when computing prefixes

[sciyoshi]

Closes #262, bug seems to have been introduced in 54d707e. Indent widths in lib2to3 for tabs are actually counted as 1, not 4, so consuming the prefix needs to match that.

[coveralls]

Pull Request Test Coverage Report for Build 792

• 9 of 9 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.04%) to 96.15%

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Totals
Change from base Build 791:	0.04%
Covered Lines:	2922
Relevant Lines:	3039

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 810

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 96.106%

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Totals
Change from base Build 791:	0.0%
Covered Lines:	2912
Relevant Lines:	3030

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💛 - Coveralls

[JelleZijlstra]

Going to close and re-open to restart CI.