vi: fix '' command (goto to prev context)

vi: fix '' command (goto to prev context)

The '' command in vi doesn't currently work because after the first
apostrophe is read, the next character is converted to an integer
between 0 and 25 inclusive (for indexing the array of marks). The
comparison of the converted character with an apostrophe therefore never
succeeds, meaning that '' doesn't do anything.

Based on the patch by Francis Rounds <francis.rounds@4bridgeworks.com>

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>