Makefile.inc1:

Makefile.inc1:
- New file.  Adds logic to search for the git binary, as well
  as determining the branch and revision, used in various
  places.

Makefile:
- Remove searching for the svn{,lite} binary.

Makefile.ec2:
- Reduce duplicated code, removing searching for the svn{,lite}
  binary, in addition to EC2_SVN{BRANCH,REV}.
- Rename EC2_SVN* with GIT* for consistency.

Makefile.mirrors:
- Remove the SRCBRANCH declaration, replaced with the exported
  GITBRANCH variable.
- Update _SNAP_SUFFIX from SVNREVISION to GITREV, and remove
  the leading 'r' from it, since it will break git hashes.
- Remove yet another instance of duplicated code to search for
  the svn{,version}lite binary.

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