fix: apply workaround patch for vcs_info (CVE-2021-45444)

This lib function applies a patch to the VCS_INFO_formats function
in zsh versions from v5.0.3 until v5.8, which don't quote % chars
in some arguments received. Normally that just means that some
% characters in these strings (branch names, directories, etc.)
will be incorrectly parsed as formatting sequences.

With CVE-2021-45444, however, this means that one of these strings
from a malicious source (e.g. a malicious git repository) can
trigger command injection and run arbitrary code in the user's
machine when visiting such git repository.

Zsh 5.8.1 fixes this vulnerability [1], but older vcs_info setups
still need a workaround such as this one to patch the vulnerability.

[1] https://github.com/zsh-users/zsh/commit/c3ea1e5d52eff8b7b172fa8c1ccc3462b43b2790

lib/vcs_info.zsh

@@ -0,0 +1,50 @@
+# Impacted versions go from v5.0.3 to v5.8 (v5.8.1 is the first patched version)
+autoload -Uz is-at-least
+if is-at-least 5.8.1 || ! is-at-least 5.0.3; then
+  return
+fi
+
+# Quote necessary $hook_com[<field>] items just before they are used
+# in the line "VCS_INFO_hook 'post-backend'" of the VCS_INFO_formats
+# function, where <field> is:
+#
+#   base:       the full path of the repository's root directory.
+#   base-name:  the name of the repository's root directory.
+#   branch:     the name of the currently checked out branch.
+#   misc:       a string that may contain anything the vcs_info backend wants.
+#   revision:   an identifier of the currently checked out revision.
+#   subdir:     the path of the current directory relative to the
+#               repository's root directory.
+#
+# This patch %-quotes these fields previous to their use in vcs_info hooks and
+# the zformat call and, eventually, when they get expanded in the prompt.
+# It's important to quote these here, and not later after hooks have modified the
+# fields, because then we could be quoting % characters from valid prompt sequences,
+# like %F{color}, %B, etc.
+#
+#  32   │ hook_com[subdir]="$(VCS_INFO_reposub ${hook_com[base]})"
+#  33   │ hook_com[subdir_orig]="${hook_com[subdir]}"
+#  34   │
+#  35 + │ for tmp in base base-name branch misc revision subdir; do
+#  36 + │     hook_com[$tmp]="${hook_com[$tmp]//\%/%%}"
+#  37 + │ done
+#  38 + │
+#  39   │ VCS_INFO_hook 'post-backend'
+#
+# This is especially important so that no command substitution is performed
+# due to malicious input as a consequence of CVE-2021-45444, which affects
+# zsh versions from 5.0.3 to 5.8.
+#
+autoload -Uz +X regexp-replace VCS_INFO_formats
+
+# We use $tmp here because it's already a local variable in VCS_INFO_formats
+typeset PATCH='for tmp (base base-name branch misc revision subdir) hook_com[$tmp]="${hook_com[$tmp]//\%/%%}"'
+# Unique string to avoid reapplying the patch if this code gets called twice
+typeset PATCH_ID=vcs_info-patch-9b9840f2-91e5-4471-af84-9e9a0dc68c1b
+# Only patch the VCS_INFO_formats function if not already patched
+if [[ "$functions[VCS_INFO_formats]" != *$PATCH_ID* ]]; then
+  regexp-replace 'functions[VCS_INFO_formats]' \
+    "VCS_INFO_hook 'post-backend'" \
+    ': ${PATCH_ID}; ${PATCH}; ${MATCH}'
+fi
+unset PATCH PATCH_ID