gitk: Make "touching paths" search support backslashes

gitk: Make "touching paths" search support backslashes

Gitk can search for commits touching a specified path. The search text is
always treated as a regular expression, regardless of the matching option
selected (Exact, IgnCase, or Regexp). In particular, backslashes escape
the next character. This is inconvenient on Windows systems, where backslashes
are the norm for path specifiers, for example when copy/pasting from
Windows Explorer or a cmd shell -- these copy-pasted paths must be manually
modified in the gitk search text edit box before they will work.

This change uses the match option "Exact" to mean that a slash is a slash,
not part of a regular expression. Backslashes are converted to frontslashes
before searching, thus allowing easy copy/pasting of paths on Windows
systems. If the previous behaviour of "touching paths" search is desired,
simply select the "Regexp" search mode.

One potential drawback is that the default setting for the match option
($findtype in the code) is "Exact", and so this change alters the default
behaviour, which may confuse users and lead to bug reports.

Signed-off-by: Yggy King <yggy@zeroandone.ca>
Signed-off-by: Paul Mackerras <paulus@samba.org>