t1300: test "set all" mode with value-pattern

t1300: test "set all" mode with value-pattern

Without additional modifiers, 'git config <key> <value>' attempts
to set a single value in the .git/config file. When the
value-pattern parameter is supplied, this command behaves in a
non-trivial manner.

Consider 'git config <key> <value> <value-pattern>'. The expected
behavior is as follows:

1. If there are multiple existing values that match 'value-pattern',
   then the command fails. Users should use --replace-all instead.

2. If there is no existing values match 'value-pattern', then the
   'key=value' pair is appended, making this 'key' a multi-valued
   config setting.

3. If there is one existing value that matches 'value-pattern', then
   the new config has one entry where 'key=value'.

Add a test that demonstrates these options. Break from the existing
pattern in t1300-config.sh to use 'git config --file=<file>' instead of
modifying .git/config directly to prevent possibly incompatible repo
states. Also use 'git config --file=<file> --list' for config state
comparison instead of the config file format. This makes the tests
more readable.

Signed-off-by: Derrick Stolee <redacted>
Signed-off-by: Junio C Hamano <redacted>