git-p4: use raw string literals for regular expressions

git-p4: use raw string literals for regular expressions

Fixes several Python diagnostics about invalid escape sequences. The
diagnostics appear for me in Python 3.12, and may appear in earlier
versions. The fix is to use raw string literals so that backslashes are
not interpreted as introducing escape sequences. Raw string literals
are already in use in this file, so adding more does not impact
toolchain compatibility.

Signed-off-by: James Touton <bekenn@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>