Fix bash math for `git bash` alias.
Fix bash math for `git bash` alias.

Previous calculation was off-by-one.

TBR=dnj@chromium.org, mmoss@chromium.org
BUG=598956

Review URL: https://codereview.chromium.org/1848793004 .

git-svn-id: fd409f4bdeea2bb50a5d34bb4d4bfc2046a5a3dd@299619 0039d316-1c4b-4281-b951-d872f2087c98

primiano/depot_tools,CoherentLabs/depot_tools,primiano/depot_tools,CoherentLabs/depot_tools,primiano/depot_tools