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.

contrib/ChangeLog

@@ -1,3 +1,8 @@
+2020-01-17  Hans-Peter Nilsson  <hp@axis.com>
+
+	* gcc_update <git revision>: Use git log "--pretty=tformat:%p:%t:%H",
+	not "--pretty=%p:%t:%H".
+
 2020-01-16  Andreas Schwab  <schwab@linux-m68k.org>
 
 	* gcc-git-customization.sh: Avoid double expansion.

contrib/gcc_update

@@ -330,7 +330,7 @@ case $vcs_type in
             exit 1
 	fi
 
-	revision=`$GCC_GIT log -n1 --pretty=%p:%t:%H`
+	revision=`$GCC_GIT log -n1 --pretty=tformat:%p:%t:%H`
 	branch=`$GCC_GIT name-rev --name-only HEAD || :`
 	;;