mergetools: drop incorrect quoting of diffargs variables

72a89cf86fcd introduced automatic quoting of diffargs in a not entirely
backwards compatible way. That rendered some of the configuration in
mergetools.rc invalid. It would fail when running extdiff on a single file with
a name that needed quoting.

Before:

  $ hg config merge-tools.meld.diffargs
  -a --label='$plabel1' $parent --label='$clabel' $child
  $ hg --config extdiff.meld= -v --debug meld
  running "/usr/bin/meld -a --label=''sp ace@0'' '.../Z.b7a65a1d2f84/sp ace' --label=''sp ace'' '.../sp ace'" in ...
  meld: error: too many arguments (wanted 0-3, got 4)

After:

  $ hg config merge-tools.meld.diffargs
  -a --label=$plabel1 $parent --label=$clabel $child
  $ hg --config extdiff.meld= -v --debug meld
  running "/usr/bin/meld -a --label='sp ace@0' '.../Z.b7a65a1d2f84/sp ace' --label='sp ace' '.../sp ace'" in ...
(success)

mercurial/default.d/mergetools.rc

@@ -7,7 +7,7 b' kdiff3.regkeyalt=Software\\Wow6432Node\\KD'
 kdiff3.regappend=\kdiff3.exe
 kdiff3.fixeol=True
 kdiff3.gui=True
-kdiff3.diffargs=--L1 '$plabel1' --L2 '$clabel' $parent $child
+kdiff3.diffargs=--L1 $plabel1 --L2 $clabel $parent $child
 
 gvimdiff.args=--nofork -d -g -O $local $other $base
 gvimdiff.regkey=Software\Vim\GVim
@@ -28,17 +28,17 b' gpyfm.gui=True'
 meld.gui=True
 meld.args=--label='local' $local --label='merged' $base --label='other' $other -o $output
 meld.check=changed
-meld.diffargs=-a --label='$plabel1' $parent --label='$clabel' $child
+meld.diffargs=-a --label=$plabel1 $parent --label=$clabel $child
 
 tkdiff.args=$local $other -a $base -o $output
 tkdiff.gui=True
 tkdiff.priority=-8
-tkdiff.diffargs=-L '$plabel1' $parent -L '$clabel' $child
+tkdiff.diffargs=-L $plabel1 $parent -L $clabel $child
 
 xxdiff.args=--show-merged-pane --exit-with-merge-status --title1 local --title2 base --title3 other --merged-filename $output --merge $local $base $other
 xxdiff.gui=True
 xxdiff.priority=-8
-xxdiff.diffargs=--title1 '$plabel1' $parent --title2 '$clabel' $child
+xxdiff.diffargs=--title1 $plabel1 $parent --title2 $clabel $child
 
 diffmerge.regkey=Software\SourceGear\SourceGear DiffMerge\
 diffmerge.regkeyalt=Software\Wow6432Node\SourceGear\SourceGear DiffMerge\
@@ -47,7 +47,7 b' diffmerge.priority=-7'
 diffmerge.args=-nosplash -merge -title1=local -title2=merged -title3=other $local $base $other -result=$output
 diffmerge.check=changed
 diffmerge.gui=True
-diffmerge.diffargs=--nosplash --title1='$plabel1' --title2='$clabel' $parent $child
+diffmerge.diffargs=--nosplash --title1=$plabel1 --title2=$clabel $parent $child
 
 p4merge.args=$base $local $other $output
 p4merge.regkey=Software\Perforce\Environment
@@ -70,13 +70,13 b' tortoisemerge.regkeyalt=Software\\Wow6432'
 tortoisemerge.check=changed
 tortoisemerge.gui=True
 tortoisemerge.priority=-8
-tortoisemerge.diffargs=/base:$parent /mine:$child /basename:'$plabel1' /minename:'$clabel'
+tortoisemerge.diffargs=/base:$parent /mine:$child /basename:$plabel1 /minename:$clabel
 
 ecmerge.args=$base $local $other --mode=merge3 --title0=base --title1=local --title2=other --to=$output
 ecmerge.regkey=Software\Elli\xc3\xa9 Computing\Merge
 ecmerge.regkeyalt=Software\Wow6432Node\Elli\xc3\xa9 Computing\Merge
 ecmerge.gui=True
-ecmerge.diffargs=$parent $child --mode=diff2 --title1='$plabel1' --title2='$clabel'
+ecmerge.diffargs=$parent $child --mode=diff2 --title1=$plabel1 --title2=$clabel
 
 # editmerge is a small script shipped in contrib.
 # It needs this config otherwise it behaves the same as internal:local
@@ -94,13 +94,13 b' beyondcompare3.regkey=Software\\Scooter S'
 beyondcompare3.regname=ExePath
 beyondcompare3.gui=True
 beyondcompare3.priority=-2
-beyondcompare3.diffargs=/lro /lefttitle='$plabel1' /righttitle='$clabel' /solo /expandall $parent $child
+beyondcompare3.diffargs=/lro /lefttitle=$plabel1 /righttitle=$clabel /solo /expandall $parent $child
 
 ; Linux version of Beyond Compare
 bcompare.args=$local $other $base -mergeoutput=$output -ro -lefttitle=parent1 -centertitle=base -righttitle=parent2 -outputtitle=merged -automerge -reviewconflicts -solo
 bcompare.gui=True
 bcompare.priority=-1
-bcompare.diffargs=-lro -lefttitle='$plabel1' -righttitle='$clabel' -solo -expandall $parent $child
+bcompare.diffargs=-lro -lefttitle=$plabel1 -righttitle=$clabel -solo -expandall $parent $child
 
 winmerge.args=/e /x /wl /ub /dl other /dr local $other $local $output
 winmerge.regkey=Software\Thingamahoochie\WinMerge
@@ -109,7 +109,7 b' winmerge.regname=Executable'
 winmerge.check=changed
 winmerge.gui=True
 winmerge.priority=-10
-winmerge.diffargs=/r /e /x /ub /wl /dl '$plabel1' /dr '$clabel' $parent $child
+winmerge.diffargs=/r /e /x /ub /wl /dl $plabel1 /dr $clabel $parent $child
 
 araxis.regkey=SOFTWARE\Classes\TypeLib\{46799e0a-7bd1-4330-911c-9660bb964ea2}\7.0\HELPDIR
 araxis.regappend=\ConsoleCompare.exe
@@ -118,7 +118,7 b' araxis.args=/3 /a2 /wait /merge /title1:'
 araxis.checkconflict=True
 araxis.binary=True
 araxis.gui=True
-araxis.diffargs=/2 /wait /title1:"$plabel1" /title2:"$clabel" $parent $child
+araxis.diffargs=/2 /wait /title1:$plabel1 /title2:$clabel $parent $child
 
 diffuse.priority=-3
 diffuse.args=$local $base $other