git.py: properly handle single-character flags with arguments
git.py: properly handle single-character flags with arguments

"git diff -U5" is correct while "git diff -U 5" is not.
ditto for "git log -U5", "git commit -F/some/path", etc.

The original version of transform_kwargs was setting up the
command line arguments such that single-character flags with
arguments were not understood by git.

This changes transform_kwargs so that the flag and argument
are both part of the same string for single-character flags.

Signed-off-by: David Aguilar <9de348c050f7cd1ca590883733c4e531ce610bf4@gmail.com>

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