rbt diff and post operations make numerous calls to 'svn info' with

paths to various files.  For any given modified file, 'svn info' 

is called 4 + N times.  This breaks down as:

3 calls from convert_to_absolute_paths() with the full filename

  to process the new file line, original file line, and Index line
1 call from find_copyfrom() for the full filename
N calls from find_copyfrom() for each sub-path component in the

  filename

By introducing caching on results from 'svn info' the number of

calls per file can be decreased to 1 + M, where M is always <= N.

For files sharing a common directory hierarchy, M will be equal

to N for the first file processed and 0 for all subsequent files.
Consider a contrived example with two modified files at:

  A/B/C/foo.txt

  A/B/C/bar.txt
The original code make 14 total calls to 'svn info'.  Measuring

timing on a CentOS 7 box with time -p reveals an average run 

time of 0.742s.  The updated code makes 5 total calls to 

'svn info' and runtime is measured at 0.586s.  This is a decrease

in runtime of 21%.

* Performed various 'rbt diff' executions with debug enabled to 
  verify expected decrease in calls to 'svn info'.
* Ran unit tests.