Use constants for git packet protocol line identifiers

Use constants for git packet protocol line identifiers

Introduce named constants for packet line headers and use them instead
of direct string literals everywhere. This not only makes the code more
readable because we don't need NON-NLS markers, it also makes it more
robust since we can use the length of these constants instead of magic
numbers.

Change-Id: Ie4b7239e0b479a68a2dc23e6e05f25061d481a31
Signed-off-by: Thomas Wolf <twolf@apache.org>