Make yadm work with Git for Windows under Cygwin

When using Git for Windows (a.k.a. msysGit) from Cygwin, git stumbles
over paths in Unix notation. Git for Windows only accepts Windows
paths (mixed notation is OK, for example `C:/GITREPO/`). This patch
converts paths passed to and from git to the appropriate notation using
cygpath if yadm is run under Cygwin and Git for Windows is detected.

yadm

@@ -37,8 +37,6 @@ CHANGES_POSSIBLE=0
 
 function main() {
 
-  require_git
-
   #; create the YADM_DIR if it doesn't exist yet
   [ -d "$YADM_DIR" ] || mkdir -p "$YADM_DIR"
 
@@ -109,7 +107,7 @@ function alt() {
   match="^(.+)##($match_system|$match_system.$match_host|$match_system.$match_host.$match_user|())$"
 
   #; process relative to YADM_WORK
-  YADM_WORK=$(git config core.worktree)
+  YADM_WORK=$(to_yadm_dir "$(git config core.worktree)")
   cd "$YADM_WORK" || {
     debug "Alternates not processed, unable to cd to $YADM_WORK"
     return
@@ -198,7 +196,7 @@ function config() {
     echo TODO: Print help about available yadm configurations
   else
     #; operate on the yadm configuration file
-    git config --file="$YADM_CONFIG" "$@"
+    git config --file="$(to_git_dir "$YADM_CONFIG")" "$@"
   fi
 
 }
@@ -208,7 +206,7 @@ function decrypt() {
   require_gpg
   require_archive
 
-  YADM_WORK=$(git config core.worktree)
+  YADM_WORK=$(to_yadm_dir "$(git config core.worktree)")
 
   if [ "$DO_LIST" = "YES" ] ; then
     tar_option="t"
@@ -235,7 +233,7 @@ function encrypt() {
   require_ls
 
   #; process relative to YADM_WORK
-  YADM_WORK=$(git config core.worktree)
+  YADM_WORK=$(to_yadm_dir "$(git config core.worktree)")
   cd "$YADM_WORK" || {
     debug "Encryption not processed, unable to cd to $YADM_WORK"
     return
@@ -273,14 +271,14 @@ function encrypt() {
   fi
 
   #; offer to add YADM_ARCHIVE if untracked
-  archive_status=$(git status --porcelain -uall "$YADM_ARCHIVE" 2>/dev/null)
+  archive_status=$(git status --porcelain -uall "$(to_git_dir "$YADM_ARCHIVE")" 2>/dev/null)
   archive_regex="^\?\?"
   if [[ $archive_status =~ $archive_regex ]] ; then
     echo "It appears that $YADM_ARCHIVE is not tracked by yadm's repository."
     echo "Would you like to add it now? (y/n)"
     read -r answer
     if [[ $answer =~ ^[yY]$ ]] ; then
-      git add "$YADM_ARCHIVE"
+      git add "$(to_git_dir "$YADM_ARCHIVE")"
     fi
   fi
 
@@ -353,7 +351,7 @@ function init() {
 
   #; init a new bare repo
   debug "Init new repo"
-  git init --shared=0600 --bare "$YADM_REPO"
+  git init --shared=0600 --bare "$(to_git_dir "$YADM_REPO")"
   configure_repo
 
   CHANGES_POSSIBLE=1
@@ -366,7 +364,7 @@ function list() {
 
   #; process relative to YADM_WORK when --all is specified
   if [ -n "$LIST_ALL" ] ; then
-    YADM_WORK=$(git config core.worktree)
+    YADM_WORK=$(to_yadm_dir "$(git config core.worktree)")
     cd "$YADM_WORK" || {
       debug "List not processed, unable to cd to $YADM_WORK"
       return
@@ -385,7 +383,7 @@ function perms() {
   #; TODO: prevent repeats in the files changed
 
   #; process relative to YADM_WORK
-  YADM_WORK=$(git config core.worktree)
+  YADM_WORK=$(to_yadm_dir "$(git config core.worktree)")
   cd "$YADM_WORK" || {
     debug "Perms not processed, unable to cd to $YADM_WORK"
     return
@@ -506,7 +504,8 @@ function configure_paths() {
   fi
 
   #; use the yadm repo for all git operations
-  export GIT_DIR="$YADM_REPO"
+  GIT_DIR=$(to_git_dir "$YADM_REPO")
+  export GIT_DIR
 
 }
 
@@ -518,7 +517,7 @@ function configure_repo() {
   git config core.bare 'false'
 
   #; set the worktree for the yadm repo
-  git config core.worktree "$YADM_WORK"
+  git config core.worktree "$(to_git_dir "$YADM_WORK")"
 
   #; by default, do not show untracked files and directories
   git config status.showUntrackedFiles no
@@ -578,6 +577,17 @@ function require_encrypt() {
 function require_git() {
   command -v git >/dev/null 2>&1 || \
     error_out "This functionality requires Git to be installed, but the command git cannot be located."
+
+  case "$(uname -s)" in
+    CYGWIN*)
+      if git --version | grep -q "windows"; then
+	use_cygpath="true"
+      fi
+      ;;
+
+    *)
+      ;;
+  esac
 }
 function require_gpg() {
   local alt_gpg
@@ -604,10 +614,28 @@ function require_ls() {
   fi
 }
 
+#; ****** directory helpers ******
+
+function to_yadm_dir() {
+  if [ -n "$use_cygpath" ] ; then
+    cygpath -u "$1"
+  else
+    echo "$1"
+  fi
+}
+function to_git_dir() {
+  if [ -n "$use_cygpath" ] ; then
+    cygpath -m "$1"
+  else
+    echo "$1"
+  fi
+}
+
 #; ****** Main processing (when not unit testing) ******
 
 if [ "$YADM_TEST" != 1 ] ; then
   process_global_args "$@"
+  require_git
   configure_paths
   main "${MAIN_ARGS[@]}"
 fi