tests/heredoctest.py

extdiff: allow a preconfigured merge-tool to be invoked

There are three ways to configure an extdiff tool:

    1) cmd.tool = (/path/to/exe optional)
    2) tool = (path/to/exe optional)
    3) tool = sometool someargs

Previously, if no executable is specified in the first two forms, the named tool
must be in $PATH, or the invocation fails.  Since the [merge-tools] section
already has the path to the diff executable, and/or the registry keys to find
the executable on Windows, reuse that configuration for forms 1 and 2 instead of
failing.  We already fallback to [diff-tools] and then [merge-tools] for program
arguments if they aren't specified in the [extdiff] section.

Since this additional lookup only occurs if an executable is not on the $PATH
for the named tool, this is backwards compatible.  For now, we assume the user
knows what he is doing if a path is provided.

This change allows a configuration file like this (assuming beyondcompare3 is
configured in merge-tools), instead of hardcoding system specific a path:

    [extdiff]
    beyondcompare3 =

import sys

globalvars = {}
lines = sys.stdin.readlines()
while lines:
    l = lines.pop(0)
    if l.startswith('SALT'):
        print l[:-1]
    elif l.startswith('>>> '):
        snippet = l[4:]
        while lines and lines[0].startswith('... '):
            l = lines.pop(0)
            snippet += l[4:]
        c = compile(snippet, '<heredoc>', 'single')
        try:
            exec c in globalvars
        except Exception, inst:
            print repr(inst)