subrepo: better error messages in _ensuregit

This patch improves the error messages raised when an OSError occurs, since
simply re-raising the exception can be both confusing and misleading. For
example, if "hg identify" is run inside a repository that contains a Git
subrepository and the git binary could not be found, it'll exit with the message
"abort: No such file or directory". That implies "identify" has a problem
reading the repository itself. There's no way for the user to know what the
real problem is unless they dive into the Mercurial source, which is what I
ended up doing after spending hours debugging errors while provisioning a VM
with Ansible (turns out I forgot to install Git on it).

Descriptive errors are especially important on Windows, since it's common for
Windows users to forget to set the "Path" system variable after installing Git.

mercurial/subrepo.py

@@ -1294,10 +1294,25 b' class gitsubrepo(abstractsubrepo):'
             self._gitexecutable = 'git'
             out, err = self._gitnodir(['--version'])
         except OSError as e:
-            if e.errno != 2 or os.name != 'nt':
-                raise
-            self._gitexecutable = 'git.cmd'
-            out, err = self._gitnodir(['--version'])
+            genericerror = _("error executing git for subrepo '%s': %s")
+            notfoundhint = _("check git is installed and in your PATH")
+            if e.errno != errno.ENOENT:
+                raise error.Abort(genericerror % (self._path, e.strerror))
+            elif os.name == 'nt':
+                try:
+                    self._gitexecutable = 'git.cmd'
+                    out, err = self._gitnodir(['--version'])
+                except OSError as e2:
+                    if e2.errno == errno.ENOENT:
+                        raise error.Abort(_("couldn't find 'git' or 'git.cmd'"
+                            " for subrepo '%s'") % self._path,
+                            hint=notfoundhint)
+                    else:
+                        raise error.Abort(genericerror % (self._path,
+                            e2.strerror))
+            else:
+                raise error.Abort(_("couldn't find git for subrepo '%s'")
+                    % self._path, hint=notfoundhint)
         versionstatus = self._checkversion(out)
         if versionstatus == 'unknown':
             self.ui.warn(_('cannot retrieve git version\n'))