shellenv: prepend colon to MANPATH if set

The current appended colon means system man pages always shadow
Homebrew's. There's also no point adding Homebrew's man dir, nor
filling out an empty MANPATH, since `man` and friends will add the
necessary dirs according to PATH.

Closes Homebrew/homebrew-core#176037.

Also fixed a syntax error in the `*csh` INFOPATH setting.

Library/Homebrew/cmd/shellenv.sh

@@ -11,7 +11,7 @@
 
 # HOMEBREW_CELLAR and HOMEBREW_PREFIX are set by extend/ENV/super.rb
 # HOMEBREW_REPOSITORY is set by bin/brew
-# Trailing colon in MANPATH adds default man dirs to search path in Linux, does no harm in macOS.
+# Leading colon in MANPATH prepends default man dirs to search path in Linux and macOS.
 # Please do not submit PRs to remove it!
 # shellcheck disable=SC2154
 homebrew-shellenv() {
@@ -33,16 +33,16 @@ homebrew-shellenv() {
       echo "set -gx HOMEBREW_CELLAR \"${HOMEBREW_CELLAR}\";"
       echo "set -gx HOMEBREW_REPOSITORY \"${HOMEBREW_REPOSITORY}\";"
       echo "fish_add_path -gP \"${HOMEBREW_PREFIX}/bin\" \"${HOMEBREW_PREFIX}/sbin\";"
-      echo "! set -q MANPATH; and set MANPATH ''; set -gx MANPATH \"${HOMEBREW_PREFIX}/share/man\" \$MANPATH;"
+      echo "set -q MANPATH; and set MANPATH[1] \":\$(string trim --left --chars=\":\" \$MANPATH[1])\";"
       echo "! set -q INFOPATH; and set INFOPATH ''; set -gx INFOPATH \"${HOMEBREW_PREFIX}/share/info\" \$INFOPATH;"
       ;;
     csh | -csh | tcsh | -tcsh)
       echo "setenv HOMEBREW_PREFIX ${HOMEBREW_PREFIX};"
       echo "setenv HOMEBREW_CELLAR ${HOMEBREW_CELLAR};"
       echo "setenv HOMEBREW_REPOSITORY ${HOMEBREW_REPOSITORY};"
       echo "setenv PATH ${HOMEBREW_PREFIX}/bin:${HOMEBREW_PREFIX}/sbin:\$PATH;"
-      echo "setenv MANPATH ${HOMEBREW_PREFIX}/share/man\`[ \${?MANPATH} == 1 ] && echo \":\${MANPATH}\"\`:;"
-      echo "setenv INFOPATH ${HOMEBREW_PREFIX}/share/info\`[ \${?INFOPATH} == 1 ] && echo \":\${INFOPATH}\"\`;"
+      echo "if ( \${?MANPATH} == 1 ) setenv MANPATH :\${MANPATH};"
+      echo "setenv INFOPATH ${HOMEBREW_PREFIX}/share/info\`if ( \${?INFOPATH} == 1 ) echo \":\${INFOPATH}\"\`;"
       ;;
     pwsh | -pwsh | pwsh-preview | -pwsh-preview)
       echo "[System.Environment]::SetEnvironmentVariable('HOMEBREW_PREFIX','${HOMEBREW_PREFIX}',[System.EnvironmentVariableTarget]::Process)"
@@ -57,7 +57,7 @@ homebrew-shellenv() {
       echo "export HOMEBREW_CELLAR=\"${HOMEBREW_CELLAR}\";"
       echo "export HOMEBREW_REPOSITORY=\"${HOMEBREW_REPOSITORY}\";"
       echo "export PATH=\"${HOMEBREW_PREFIX}/bin:${HOMEBREW_PREFIX}/sbin\${PATH+:\$PATH}\";"
-      echo "export MANPATH=\"${HOMEBREW_PREFIX}/share/man\${MANPATH+:\$MANPATH}:\";"
+      echo "[ -z \"\${MANPATH}\" ] || export MANPATH=\":\${MANPATH#:}\";"
       echo "export INFOPATH=\"${HOMEBREW_PREFIX}/share/info:\${INFOPATH:-}\";"
       ;;
   esac