IMPALA-4096: Allow clean.sh to work from snapshots
IMPALA-4096: Allow clean.sh to work from snapshots

buildall.sh calls bin/clean.sh, which fails when not in a git
directory. This skips the "git clean" steps when not in a git
checkout.

Todd Lipcon found this when testing a snapshot created using git
archive.

Change-Id: I12dd9035298151557491009680d66d25c8f58c1d
Reviewed-on: http://gerrit.cloudera.org:8080/4336
Tested-by: Internal Jenkins
Reviewed-by: Jim Apple <9e3620e7251463d214b5f536438d2c9ea899162d@cloudera.com>

cloudera/Impala,michaelhkw/incubator-impala,cloudera/Impala,cloudera/Impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,cloudera/Impala,cloudera/Impala,cloudera/Impala,michaelhkw/incubator-impala,cloudera/Impala,michaelhkw/incubator-impala