config: require at least one digit when parsing numbers

config: require at least one digit when parsing numbers

If the input to strtoimax() or strtoumax() does not contain any digits
then they return zero and set `end` to point to the start of the input
string.  git_parse_[un]signed() do not check `end` and so fail to return
an error and instead return a value of zero if the input string is a
valid units factor without any digits (e.g "k").

Tests are added to check that 'git config --int' and OPT_MAGNITUDE()
reject a units specifier without a leading digit.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Taylor Blau <me@ttaylorr.com>