Fix recognition of reviewers in commit message tags

A bug in JGit's footer match method caused "Sample-Bug-Id: 42"
to be treated identical to the footer line "Signed-off-by: 42".

Since Signed-off-by tags are converted into reviewers, we were adding
account number 42 as a reviewer anytime we saw "Sample-Bug-Id: 42".
This happened for any footer line key, so long as its length matched
the length of "Signed-off-by".

This was fixed in a newer version of JGit, so upgrade.

Also prevent "Signed-off-by: 42" from being treated as an account.
Within a commit message, we only want to match to a reviewer by
their full name or their email address.  Matching on their internal
database id number isn't useful.

Bug: issue 553
Change-Id: Ie26974240f26d29fb55cd95dc6b6c72c89903f37
Signed-off-by: Shawn O. Pearce <sop@google.com>
4 files changed