contrib/gcc_update: Insert "tformat:" for git log --pretty=tformat:%p:%t:%H

Really old git versions (like 1.6.0) require
"git log --pretty=tformat:%p:%t:%H"
or else we see:

Updating GIT tree
Current branch master is up to date.
fatal: invalid --pretty format: %p:%t:%H
Adjusting file timestamps
Touching gcc/config.in...
Touching gcc/config/arm/arm-tune.md...

...and an empty revision in LAST_UPDATED and gcc/REVISION.
In its absence, for newer git versions, "tformat" is the default
qualifier, documented as such default for at least git-2.11.0.