test-merge-tools: stabilize for Windows

This masks the Windows argument parsing insanity[1], so it needs a bit of
explanation.  (The security reference in the footnote is probably useful to keep
in mind if we ever whitelist certain in-repo config settings.)

9037c29e9f53 introduced tests that were failing on Windows with an unbalanced
double quote[2].  What ends up happening here is util.shellquote() is double
quoting the file path, but the shell script is placing this ->": "<- right next
to it.  So cmd.exe gets launched with 'lb:base": ""c:\...\f~base.xyz"', which
got interpreted as 'lb:base: "c:\...\f~base.xyz'. If the test is adjusted to
quote like "lb:$labelbase: $base", then MSYS runs interference and strips the
'\' characters.  I was able to get the expected result by dropping the quotes
from '": "', and changing the space to underscore.  But since we need to glob
away the C: part anyway, just glob away the quote and leave the test unchanged.

[1] https://blogs.msdn.microsoft.com/twistylittlepassagesallalike/2011/04/23/everyone-quotes-command-line-arguments-the-wrong-way/
[2] https://buildbot.mercurial-scm.org/builders/Win7%20x86_64%20hg%20tests/builds/441/steps/run-tests.py%20%28python%202.7.13%29/logs/stdio

tests/test-merge-tools.t

@@ -1358,7 +1358,7 b' mergemarkertemplate settings:'
   arg: "ll:working copy"
   arg: "lo:"
   arg: "merge rev"
-  arg: "lb:base: /*/f~base.*" (glob)
+  arg: "lb:base: */f~base.*" (glob)
   0 files updated, 1 files merged, 0 files removed, 0 files unresolved
   (branch merge, don't forget to commit)
   $ rm -f 'printargs_merge_tool'
@@ -1388,7 +1388,7 b' mergemarkertemplate:'
   arg: "ll:working copy: tooltmpl ef83787e2614"
   arg: "lo:"
   arg: "merge rev: tooltmpl 0185f4e0cf02"
-  arg: "lb:base: /*/f~base.*" (glob)
+  arg: "lb:base: */f~base.*" (glob)
   0 files updated, 1 files merged, 0 files removed, 0 files unresolved
   (branch merge, don't forget to commit)
   $ rm -f 'printargs_merge_tool'