Added build option SONAME to control whether VERSION and SOVERSION were set for the git2 target, as some platforms require that this NOT be set.
refs: explicitly catch leading slashes

It's somewhat common to try to write "/refs/tags/something". There is
no easy way to catch it during the main body of the function, as there
is no way to distinguish whether it's a leading slash or a double
slash somewhere in the middle.

Catch this at the beginning so we don't trigger the assert in
is_all_caps_and_underscore().
Merge pull request #1401 from carlosmn/leading-slash

refs: explicitly catch leading slashes
immutable references and a pluggable ref database
Merge remote-tracking branch 'ethomson/immutable_refs' into development
refs: Dude, you're OUT.
optional tracing
Merge pull request #1403 from ethomson/tracing

Optional tracing back to consumers
Merge pull request #1399 from nathan-osman/development

Add SONAME build option to facilitate building for Android.