mergetool: use path to mergetool in config var mergetool.<tool>.path

mergetool: use path to mergetool in config var mergetool.<tool>.path

This commit adds a mechanism to provide absolute paths to the
external programs called by 'git mergetool'. A path can be
specified in the configuation variable mergetool.<tool>.path.
The configuration variable is similar to how we name branches
and remotes. It is extensible if we need to specify more details
about a tool.

The mechanism is especially useful on Windows, where external
programs are unlikely to be in PATH.

[sp: Fixed a few minor issues prior to applying]

Signed-off-by: Steffen Prohaska <prohaska@zib.de>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>