automake: increase the MESA_GIT_SHA1 hash id length from 7 to 10 digits

automake: increase the MESA_GIT_SHA1 hash id length from 7 to 10 digits

The SCons build has been using 10 digits of the git hash id for the
MESA_GIT_SHA1 string in git_sha1.h for about a year now.  I bumped it
up after running into a case where a 7-digit hash ID was ambiguous.

This patch makes the same change for the autotools build.

The command "git log | grep "^commit" | cut -b 8-14 | sort | uniq -d"
shows there are currently 17 cases where 7 digits of hash id are
ambiguous on master (probably quite a few more if we'd consider other
branches).

Instead of using "git log -n 1 --oneline" use
"git rev-parse --short=10 HEAD" to get the HEAD hash id.

v2: use printf instead of sed, per Eric's suggestion.

Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>